Here's some food for thought...
Both, in my opinion, are belt guns... Given your intended uses, I'd say go with the GP100 or find a good used S&W Model 66.
The SP101 is smaller, but not small, and has 5 shots. 357 out of these guns is tolerable but not fun.
The GP100 is more versatile, carries 6 rounds and hot 357 mag loads are easy to shoot from this frame size.
Another consideration is the S&W 66, especially a good used one in 3" or 4". Very similar in size to the SP101 but with 6 rounds and its a real classic in terms of weight and balance. The trigger on the 66 is usually much better than a Ruger due to the leaf spring vs the coil spring.