It just depends on the combination. Generally roundness is good but for some types other properties can be more beneficial - in the case of Master Draciel's Smash Turtle, it has some force smash at the edges of it's wings, is fairly round and the nubs are supposed to grind stamina off the opponent, though the force smash is far more noticeable. The force smash slopes backfire unless the bey they're on is heavy/resistant enough (I discussed this in plastic parts discussion), but weight based defense is basically the epitome of "heavy/resistant enough". That said, Master Draciel's AR doesn't have much recoil. Protruding AR's do create a weakness to upper attack but Master Draciel's AR doesn't affect that on weight based defenders.
Lowering recoil is actually more notable for survival types where there's nothing else compensating for it and no benefit to extra smash, but two types of defense benefit greatly from survival properties, sooooo.
For plastics, you have to lose the MFB Mindset - the three types of defense are all very distinct. In fact, the various sub-types of each main type are all like that, it makes writing a competitive combos list very difficult.
Defensive Zombies share most requirements with Zombies, therefore round parts are a priority, though weight is equally important, so wide defense is much better than the rounder but much lighter wide survivor, while no other WD's are round enough to be useful (though MG WD can be used with Cross Survivor SP's with some effectiveness, this setup has much, much poorer survival).
For Wolborg 4 based defense setups, the AR has little impact on the defense as long as it doesn't protrude to offer contact points to the opponent, so the focus there is on increasing the survival ability, though defense and survival have similar traits: basically, minimising recoil is important for both. If you're using 10 balance instead of wide defense, taking a more defensive approach with the AR can be a good idea.
And then there's weight based defense, which we've discussed. Tiger Defenser and co are by no means useless for it, they're still top-tier choices, but Master Draciel's is much more versatile. Venusian G's AR is also pretty decent, in left spin it has rather low recoil and it's also really heavy, but generally I find the survival tradeoff isn't worth it.
Also, a little note, while Galeon's AR is touted for its defensive ability, it's improvement over Tiger Defenser is pretty minor. The shape is less protective and has more recoil, while the SAR compensates for this, it also has worse stamina. Also, it's fragile as heck, especially if you aren't using a setup that shields it somewhat (i.e. wide defense/survivor), and even then the main AR breaks fairly easily. It's not a bad AR by any means (and the SAR is fantastic), but personally I find tiger defenser much more useful.