Im leaning towards the GP100, kinda reminds me of the S&W L frame. I have a weakness for stainless 4" barreled revolvers. would have been nice if it had a half-underlug.
If you like 4" stainless half lug revolvers, you are describing the Ruger Six Series in Stainless and the S&W Model 66 to a T. I don't have pictures of either, but skidder's picture above (top revolver) is a very nice 4" Security Six. I do own examples of both, and greatly enjoy both platforms, though I have a slight preference for the S&W K Frames.
The Ruger Security Six (square butt, adj sights), Police Service Six (square butt, fixed sights), Speed Six (round butt, fixed sights) and the S&W Model 66 K frame are all out of production, but they can be found fairly easily on the used market if you are patient. These 6 shot revolvers neatly split the difference in size between the GP100 and SP101. The Ruger Sixes are more robust, while the K Frame Smiths are generally more refined.
Your choice, just be sure it passes the revolver checkout. Interestingly, even though Rugers are more robust, on the used market I've found most are shot to total crap. That "built like a tank" mantra regarding Rugers seems to encourage people to abuse the mess out of them with atomic loads. They are well built, but they are certainly not indestructible. Consequently, I find it much easer to find a well cared for, mechanically solid S&W than a Ruger. Go figure.
The S&W K Frame will hold up fine, just stick to shooting .38s or 158 grain .357s (no 125 grain .357 loudenboomers).