The T3S coils are the 'universal donor' - they fit all - evod, protank, mini protank, MT3S (of course)... however the evod/protank coils don't fit the T3S/MT3S.
This guy says it best as to why:
http://www.e-cigarette-forum.com/fo...more-economical-change-heads.html#post9999615
"No, the Protank/Evod heads can't fit in the T3 tank, the flange under the wick of the head is wider than the T3s head. They screw down on the T3s base, just the base doesn't fit in fully down with these heads in them. So my suggestion is get spare T3s heads, which I am doing then rebuild them. Upgrading all my older T3 tanks to T3s, as I have been rebuilding them, the removable head makes it easier to cut off the excess wire legs of the neg and pos compared to the standard T3 it is hard to get even small flush cutters down in the cup and get majority of the excess. Beside that the T3s/MT3s performs just as competently as the standard T3/MT3."
Basically the T3S/MT3S coils are easier to rebuild and are cheaper and fit on all bcc kanger products for the eGo.