I have both and still can't decide which I like better. My SP101 is a 3" and carries quite handily. The only time I've felt under gunned is in the southwest mesas in cougar and black bear country, when in the sheep camp. This is the main reason I also picked up a 4" GP100, so now I carry a "brace" of pistols. Now having carried over many years I think I would break it down like this:
2" SP101 - EDC concealable belt revolver.
3" SP101 - EDC CCW or general use kit/pack revolver
3" GP100 - can be concealable..general purpose kit gun.
4" GP100 - I have boot grip, and can conceal...if I have to (but probably never will). General purpose can do anything personal sidearm.
If I were only limited to one gun, hesitantly, I would say the GP100 with a 4" barrel. I can still conceal if I absolutely have to, but the SP101 is so much nicer for this.
You said another 357. If you don't mind me asking, what do you currently have? Are you looking to cover a niche you don't already, or add another one for redundancy?