"I would carry my G26 or G19"
if you really want a Glock revolver, buy one (???)
hint = "I want something to replace my XDm for warmer weather"
GP is a tad large for IWB, yes
snubbies not a lot of fun with 357 mag full loads, and snubbie barrels do very little extra with magnum load powder
the SP101 is one that handles it better than most, especially in 3" barrel
"I discovered with my Mom's SP101...the magnum rounds hurt my hand a bit,
yes, but...
"What's wrong with carrying and shooting 38 Specials?"
not a thing, and 38+P suits 3" barrels real well, far better than 357s do
"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."
YES, it does, but 'tis a tad large/heavy for IWB
"So...anything on the spur question?"
minor relevance, hammer spurs not friendly to IWB (under shirts), matters significantly less for OWB.. BUT it's really not that big a deal to take the "spur" off for DAO (any DA/SA revolver), as was noted above
"which I could get with the spurless hammer".. yes
PS
no offense intended WARP, but the OP asked 4" Ruger GP revolver vs 3" Ruger SP revolver, not Glock vs XDm