Nothing but one shooter's opinion-
If I want a 4-6" 357 revolver, I sure do like the GP100. If I'm to get any smaller than that, I'll go ahead and go with the SP101. Basically it's just a mini GP100, but scaled down throughout to make it more compact for carry. Still plenty big for the average guy to get 100% of his hand around though.
Try the grip on each and take into consideration your own intended use (carry, range, hunting sidearm, etc). From my own perspective, I'd see little use in going shorter than 4" for anything but carry...hence the SP101 preference in my case.