A couple of things, the Istick 100w has a new software upgrade that will allow it to go up to 133w, meaning to factor that in she choosing batts, and I doubt you'll need much more power, it's a nice sized beast! I went with the Istick 200w merely because it was on sale for less than the 100w and I thought the extra runtime would be nice while on vacation.
Now for batteries, I'm a bit of a battery Nazi! If you get a multiple battery mod, label them in sets, two, three, whatever, in a manner YOU know how they go, and keep them married! Charge and discharge them equally and keep them together during their lifespan. Do not use them in other mods, do not break up the pairing mixing and matching them etc. Those batteries go with that mod until they die!
Secondly, unless you are planning to push things to the limit all the time, I'll suggest the Samsung LG HG2's for the runtime, that's what I got for my Istick 200w. Now if you want to crank it and amp it out blowing clouds everyday, get the Sanyo 25R's or Sony VCT4's for the higher amperage.