Lighter = Better Stamina is absolutely wrong - a misconception from the era of plastics due to the wider weight disks generally being light.
There may be some truth to it for spin-stealers with regards to lowering inertia, but the tradeoff in terms of stamina/resisting the slowdown from friction that comes with that seems to be much more significant for most beyblades - it depends on the amount of friction and so on and the only setup I can think of that might be benefiting from it is Spiral Change Base (which seems to be very low friction/is making up for it with that ludicrous LAD). However, this assumes a similar weight distribution - centred weight also lowers your inertia, though again this is a matter of how centred and how much weight we're talking about, as this affects two variables in the moment of inertia equation (the total mass, and how far the centre of mass is from the central axis). Working that out could give a solution but you'd need to work out where the centre of mass lies on each beyblade and a bunch of other stuff to work it out - quite a lot of work.
Basically, lighter isn't best in and of itself, it's about the weight distribution - and M(S)F-H/M(S)F-M are right in the centre of a beyblade.
Having played around with the heavier and lighter Circle Wides in HMS Zombies, the heavier one works better and wins 1v1, but at the same time I'm not going to run off and use Circle Heavy (sometimes CWD's make up for the tradeoff in defensive attributes, especially with Circle Upper which has the weight distro to compensate for this, but that's getting away from the point). I think that enough of a weight advantage over opposing combinations means your distribution doesn't matter that much (see Basalt's success) but when you're dealing with weight right in the centre of a beyblade, that's going take a whooooole lot of weight to make up for what you're losing, and metal faces aren't going to be heavy enough - maybe on light wheels, but not on any serious MFB.
This is why I bought up Spiral Change Base - even with the amount of weight an HMC adds to what is a very light combination, it still doesn't make up for the fact it is right in the centre - the combination is rendered useless by its addition, even though it adds plenty of weight, the spin stealing and/or LAD and/or stamina are hurt too much by its location. This is basically what we're looking at here, and why I chose it for my example.
tl;dr: lighter =/= better stamina, it's all about distribution, which is really, really important to both stamina and spin stealing and is the main thing we're looking at here. Similar cases indicate strongly that centred weight is bad for stamina and while basalt's success and the general poor performance of plastics spin stealers vs even pretty mediocre MFB indicates that if it's a very significant amount of weight then it could compensate for it (which I am almost 100% sure drops drastically as weight is added further from the centre, hence the performance differences we've seen with Phantom (and what I believe to be the cause of the discrepancies with Metal Fury Scythe in limited)), faces are nowhere near a significant portion of an MFB's weight, so even the heaviest won't compensate for it.
Keep in mind I'm not 100% sure on all of that but at the least, I'm confident that a lighter face is better for spin-stealing (and almost certainly stamina) based on very similar situations.
By the way, I think we should add B: D to Duo, on the grounds it's more forgiving with weak launching than EWD, and taking hits from the major opponents Duo stamina is used against isn't really any more of an issue for it than it is for SA165EWD. It also outspins 145-height stamina in my experience. FWIW, my B: D spins really, really freely, should do a solo spin with it but yeah, that could well be part of it, in fact it seems like it might be better against F230 than SA165EWD does, though that's almost certainly because my 'testing' EWD is getting on a bit (not aggro, but it doesn't spin as long as my fresher one). Still not good enough to go on Dragooon, SA165 is much better there, but for Duo, it's good enough to deserve a spot IMO.
There may be some truth to it for spin-stealers with regards to lowering inertia, but the tradeoff in terms of stamina/resisting the slowdown from friction that comes with that seems to be much more significant for most beyblades - it depends on the amount of friction and so on and the only setup I can think of that might be benefiting from it is Spiral Change Base (which seems to be very low friction/is making up for it with that ludicrous LAD). However, this assumes a similar weight distribution - centred weight also lowers your inertia, though again this is a matter of how centred and how much weight we're talking about, as this affects two variables in the moment of inertia equation (the total mass, and how far the centre of mass is from the central axis). Working that out could give a solution but you'd need to work out where the centre of mass lies on each beyblade and a bunch of other stuff to work it out - quite a lot of work.
Basically, lighter isn't best in and of itself, it's about the weight distribution - and M(S)F-H/M(S)F-M are right in the centre of a beyblade.
Having played around with the heavier and lighter Circle Wides in HMS Zombies, the heavier one works better and wins 1v1, but at the same time I'm not going to run off and use Circle Heavy (sometimes CWD's make up for the tradeoff in defensive attributes, especially with Circle Upper which has the weight distro to compensate for this, but that's getting away from the point). I think that enough of a weight advantage over opposing combinations means your distribution doesn't matter that much (see Basalt's success) but when you're dealing with weight right in the centre of a beyblade, that's going take a whooooole lot of weight to make up for what you're losing, and metal faces aren't going to be heavy enough - maybe on light wheels, but not on any serious MFB.
This is why I bought up Spiral Change Base - even with the amount of weight an HMC adds to what is a very light combination, it still doesn't make up for the fact it is right in the centre - the combination is rendered useless by its addition, even though it adds plenty of weight, the spin stealing and/or LAD and/or stamina are hurt too much by its location. This is basically what we're looking at here, and why I chose it for my example.
tl;dr: lighter =/= better stamina, it's all about distribution, which is really, really important to both stamina and spin stealing and is the main thing we're looking at here. Similar cases indicate strongly that centred weight is bad for stamina and while basalt's success and the general poor performance of plastics spin stealers vs even pretty mediocre MFB indicates that if it's a very significant amount of weight then it could compensate for it (which I am almost 100% sure drops drastically as weight is added further from the centre, hence the performance differences we've seen with Phantom (and what I believe to be the cause of the discrepancies with Metal Fury Scythe in limited)), faces are nowhere near a significant portion of an MFB's weight, so even the heaviest won't compensate for it.
Keep in mind I'm not 100% sure on all of that but at the least, I'm confident that a lighter face is better for spin-stealing (and almost certainly stamina) based on very similar situations.
By the way, I think we should add B: D to Duo, on the grounds it's more forgiving with weak launching than EWD, and taking hits from the major opponents Duo stamina is used against isn't really any more of an issue for it than it is for SA165EWD. It also outspins 145-height stamina in my experience. FWIW, my B: D spins really, really freely, should do a solo spin with it but yeah, that could well be part of it, in fact it seems like it might be better against F230 than SA165EWD does, though that's almost certainly because my 'testing' EWD is getting on a bit (not aggro, but it doesn't spin as long as my fresher one). Still not good enough to go on Dragooon, SA165 is much better there, but for Duo, it's good enough to deserve a spot IMO.