I'm not sure what you consider 'hotish' .45 Colt loads, but if you look at both, the Ruger has a thicker cylinder and top strap, but is heavier than the Smith.
If you are talking about loads like Buffalo Bore or Double Tap, I would think the Ruger will stand up better in the long run. Nothing wrong with the Smith, but when you are shooting those kinds of loads, you want as much metal around you as possible.
Both guns are based off their .44M models and the cylinders and frames are the same for the .45 Colt as the .44M, just bigger holes. This makes the cylinder walls thinner. Ruger started out with a bigger cylinder than Smith so the walls and frame are thicker. If you plan to carry a lot and shoot little the Smith should do you fine. If you plan to shoot the 'hotish' .45 Colt loads on a regular basis, then I would go with the Ruger.
The Mountain Gun is the same frame and cylinder as the other N frames, but has a lighter contour barrel and 'black powder' bevel on the cylinder and a few other enhancements to lighten the weight. I have a MG in .44M and it is a delight to carry, but I have the Hogue X-frame grips so I can shoot the thing. Hope this helps----