Why one or the other? Several of each! The SP101 is a bit larger and heftier than an S&W J-frame, so it will behave much more like a true fightin' revolver. A GP100 undoubtedly IS a big, hefty fightin' sixgun, but one with a 4" barrel can be concealed amazingly well.
Hammer spurs? I carried two SP101 snubbies to the grocery store earlier. One had a hammer spur, and the other no spur. Yes, within sight of the building housing the Israeli consulate, and within the shadow of an energy/banking center, I will indeed carry two snubbies, and actually, that was light. I often carry one snubby and something bigger on such occasions.
If it came down to having only one or the other, for all purposes, GP100 is my choice. I actually carry my SP101s more often.