This is like asking if you should buy a Peterbilt or a VW Bug. Do you want to haul things, or commute?
Are you a collector, or a shooter? If you want to shoot a lot of magnums, the heavier gun will probably be more pleasant, over time. I don't understand collecting, so I won't go there.
Do you need a concealed carry piece? I can certainly hide a 4" GP100, but others insist that an SP101 is too big to conceal. Neither gun is light for its size envelope, but I don't mind the density and heft, and am used to sturdy belts and holsters. Some folks DO mind the weight and heft.
How big are your hands and fingers? I have skinny enough fingers, that I can get all of my fingers on the factory grip of an SP101. I can lock the SP101 grip into my hand, so that there is amazingly little kick. Your hands may vary.
I own several of each one of these guns. The GP100 is a superb utility sixgun. The SP101 is a superb compact carry piece. Two different roles.