With the S2 you can hold both eyes open while shooting. With the SP you only think all the time about the other lenses you can use. While shooting, it is better to think about pictures.
I wonder where the hell you found that idea of "thinking all the time about the other lenses you can use" : with the 50mm framelines selected, the SP does not display anything else than the 50mm framelines for instance (with continuous parallax compensation by the way, something the S2 does not). So if you use the only lens which the S2 offers framelines for, i.e. the 50mm, the SP finder will display the exact same thing as the S2 finder, with the same global greenish tint and the same yellowish RF patch. Of course the framelines of the SP finder have rounded corners, like the Leica M3 framelines. So what ?
The only serious answer is : it all depends on the actual condition of the viewfinder you look through. My 1962 SP has a stellar viewfinder, and is better (global brightness, RF patch contrast, resistance to flare) than both the finders of my two 1954 and 1956 S2 bodies, although those are just as good as it gets for any clean and desilvering-free S2 finder.
My 1958 S3 has an excellent viewfinder. It doesn't flare the way many people despise the S3 for. And the main advantage of it over my S2 finders is that, since I have to wear glasses all the time, I can see the 50 mm framelines way more easily. I don't see the 35 mm framelines - which is normal when you wear glasses - and frankly the always visible 105 mm framelines are something I never think of. After a while, if you never use the 105 mm, just because you don't own any, the 105mm framelines are something you will never notice.
I can keep both eyes open with the three models and none distracted me from thinking about pictures, ever.
I have traveled a lot with my S3 only. I never thought I would have got better pictures if I had taken the S2 instead.
Like one of the previous posters, I would say that the condition of my SP does that I feel a bit reluctant to carry it everywhere, especially on travel or as my 24/24 camera. I should use it more, sure.
Bottom line : a clean vintage S3 finder is better than the reissue S3 finder. The framelines of the reissue finder are too thick and, although new, that finder can flare a lot sometimes.
To the OP : with my Nikon rangefinders I use lenses from 21mm to 50mm. For longer lenses I use my Nikon SLRs and my F-mount Nikkors.