The difference is that protanks 1 & 2 (including minis) are single coil while protank 3 (both regular and mini) are dual coil.

Since I prefer single coils (protanks 1/2) but like the looks of the mini protank 3, I've tried putting the mini protank 2 head in the mini protank 3. It fits fine, fires up, but it's prone to some leaking at the coils (since you now have a single coil occupying the slightly larger space the dual coil used to occupy). If you lay a piece of cotton or another wick on top of the single coil, it seems to help.