Duotrons are notoriously terrible, even for consistency, haha.
Personally, for consistency I generally like a well-functioning LL2 with a launcher grip but that's not a common thing. I can't think what else would do well, and if you're testing something it's generally best to stick with the 'real deal' to emulate the conditions they are used in as best as possible, and just try to focus on using the same power for each launch.
I forget how accurate the rip gauge launchers are, but you could look into something like that, or, if you wanted to get really scientific and crazy about it, do something involving a motor and have it cut at a particular RPM, so you have the same thing each time. (And then rig it up to produce some ridiculous RPM and record the resulting battles for youtube).
Generally though, just use a regular beylauncher and get a feel for it and try to stick to that