This is a tough one...and, despite what my handle indicates, I'm going to pick the GP100.
Here's why: I have a Springfield 1911, the bare bones GI model, and I like it fine. But, every time I go to the range I think to myself, 'I like firing this gun...can't wait till I can get a Les Baer/Wilson Combat/SA TRP (fill in the blank).' I like the 1911 platform: how it works, how it feels in my hand, how it breaks down and goes back together, and what a nice big round it puts down range. All that said, I like shooting the GP100 better. It's accurate as all get out, fits well in my hand, and is just a pleasure to shoot full magnum loads with.
In conclusion, I feel I need to upgrade my 1911, not my .357 magnum.