The SP-101 is probably the way to go for carry. The GP100 wins out by a mile in every other situation. I have a 2.25" SP101 and it's a great little gun. Quite a handful with full-house loads though. I don't carry it so I'll probably trade it off at some point for a GP100 or S&W 627 or 686. They're all excellent guns.
I've been eyeing one of those 4.2" SP-101's as well though...stainless with the wood-inlay grips (love those grips)...a beautiful gun in my opinion. I hate the size:capacity ratio of that particular setup. 5 rounds in a relatively full-framed and heavy gun seems a bit impractical, but it would probably just be a range toy anyway.
But back to the OP: If it's simply for range work and home defense, you want the GP100. Great gun. Beautiful, simple, reasonably priced, and built like brick sh!thouse.