They are all 3.7
devices. All 3 run at the same voltage.
The mini vapestack uses the smallest battery (16340) of the 3 so it will have the shortest run time but is available in colors. It will run about 5x longer than a stock e-cig battery.
The midi and roughstack both use the same 18350 battery and will last about 1.5 times longer per charge than the mini. The main difference between the midi and the roughstack is looks. The roughstack was designed as a "don't worry about scratches and dings"
mod, where the midi just looks really nice.
If I were in the market for a smaller sized 3.7 mod, the Midi- Vapestack would be my #1 choice.
Actually, if it could run 6v, I'd buy one today.. lol
The mods at Mad Vapes are IMHO the best deal going right now for 3.7v mods.