[BrianG]

Well, with a kv of 3100and using the MM, I assume you meant to put the 3s packs in parallel? Otherwise, the total rpm would be kinda high. Assuming yes, two 3s 5Ah packs in parallel nets a total of 10Ah capacity and 200A discharge rating (5Ah X 20C X 2 packs). Yes, that would be fine. I would prefer that over the 4s 4Ah 25C setup. Even on 3s, I would use a UBEC on the MM.

Do you have the motor, ESC, batteries yet? If not, I would suggest something a little different, depending on the tranny setup.

If using the two speed tranny, get a Medusa 36-60-1600, dual 3s 5Ah 20C packs in series (total of 6s) and the MMM instead. You can get away with a smaller motor since the two speed will help with low end torque and top speed.

If using a single speed tranny, I would opt for the Medusa 36-70-1600, dual 3s 5A 20C packs in series (6s total), and the MMM. The taller gearing of the single speed will work better with the 70mm Medusa.

Either way, I'd go for the MMM because; 1) it supports higher voltage and it's better to use higher voltage and lower current for a given power, 2) it has a built-in BEC, which would be about the same price as a MM and a seperate BEC, and 3) the MMM handles more current than the MM.