I just bought a GP100 3" on gunbroker.com from Wilson and Associates for $411 shipped. Should have it either end of this week or early next week. I have only seen the 4" in person, as I can't find any 3" locally to look at. But in all the pics I've seen I like the 3" as far as looks go. Most people who have 3" revos also comment frequently that the gun has good balance as well. I'm going to sell my S&W model 66 4" to a friend who doesn't have any guns and is starting to get into shooting.
I plan on shipping it to Clark Custom Guns for sure for an action job, chamfer the cylinder holes, and change out the front sight for something else. I also may have them put in the white outline rear blade sight (I know this would make it tougher to conceal, but this is going to be a nightstand/fun at the range gun). I may also have them polish off the Ruger stamping on the barrel left side, and the big "GP100" on the right, they said it would cost about $75 to do this. I know others have done it, but if I'm already spending over $600 on the gun and work, I might as well have Clark do it right.
If you plan on concealed carry I would go with the SP101, but if its for range/home protection I would get the GP100.