The power of a full house .357 out of a heavy 4" barrel gun would seem to trump whatever you put in a 2.25" barrel. That is one reason to consider a GP100, I'd say. I haven't IWB carried either so I don't really know.
Ah yes, the voice of inexperience. Since I primarily carry (and shoot, barring .22lr, of course) .357 magnum, I can tell you that there are
factory loads that will do close enough from a 2.25" bbl vs a 4" bbl as doesn't matter. I carry Georgia Arms 158 gr Speer Gold Dots, and I've chrony'd them at 1150 from my 2.25" SP101 vs. 1250 from my 4" GP100. The 158 gr SJHP Remington's are pretty similar.
Also, if you haven't carried either gun, why are you trying to advise the OP on carrying either one? That's not helpful.
As far as the hammer issue goes for IWB carry, I carry my SP101 at my appendix. This is probably the
worst location with regard to the hammer digging into my belly. Guess what: it's not an issue. I experience more discomfort from the rubber grips chafing my skin, and that's easily alleviated by wearing an undershirt. Same goes for the 4" GP100, except that the barrel typically pokes into my thigh, and the grips jab my ribs. Neither of these are an issue with the smaller SP101.
I carry the SP101 in one of these:
http://www.elitesurvival.com/index.php?p=product&id=28
I think it's a size 9, but I've had it so long that the writing on the tag has faded. The holster material is still strong, though.
I can also carry the GP100 in that rig, if need be. However, I prefer the Simply Rugged Pancake with IWB straps. Well worth the cash!
I used to own a Glock 27. I carried it in an IWB rig similar to what I use for the SP101, as well as a Fobus holster. Didn't like it, so I sold it. Too thick and blocky, printed too much, and performance wasn't what it should have been from the 3.5" bbl.
Also, the GP100 is more concealable than the Glock 27 FOR ME. Fits my body better, whereas the Glock just wants it's sharp little edges to stick out all Teutonic-like and whatnot. The SP101 is waaaaay more concealable than the Glock 27 FOR ME, and more concealable than the GP100.
Just FYI, OP, from personal experience and experimentation.