The S2, S3, and SP (and S4, for purists) all have 1:1 finder. It is one of the most important parts of the Nikon system, as far as i'm concerned. The viewfinder becomes a window on the real world, not a microscope view of a miniature world.
The S2 is louder. It uses a rugged shutter that makes a definite "ker-thwack!" sound that is not stealthy or rangefinder-like. The SP/S3 shutter was redesigned and is rangefinder-quiet. The single 50mm frameline on the S2 makes it very suited to 50mm photography. It has a raised-ridge ring around the eyepiece that makes it a glasses scratcher even for 50mm lenses. The S3/SP changed this design to be more flush with the back of the camera.
The s2's 2-speed shutter takes getting used to. You have to raise the top of the high-speed dial to change speeds, and one of the settings allows you to activate the 1-30 speeds on the lower dial. Also, the frame counter is a more basic "set it yourself" design, where you have to remember to reset to zero after changing film, versus the automatic counter on the SP and S3. The S2 is a little heavier. The X-sync is about 1/45 or 1/50 and so has its own speed setting.
The SP has a more modern layout, identical to the F, that really helped set the standard for controls on a modern camera. I don't find the SP viewfinder to be cluttered (i like the "tunnel effect" of the frame lines.) However, it takes a bit of time to get used to the separate wide-angle finder. For a newcomer, this might be better described as "distracting" than cluttered. When a waiter or passerby offers to take a group photo of us, they invariably get a very puzzled look and just center the camera using whichever finder their eye has discovered first. In this respect, it truly is a "professional" camera -- it has to be explained. Overall, the SP finders are a little dimmer than the S2 or S3. They are finders most likely to have a washed-out RF patch, rendering them almost unusuable.
As noted above, the S3 is an excellent camera. In certain respects, I prefer it to the SP, and it is under-appreciated to the extent that it is always far more affordable than the SP. I prefer the S3 for 50mm, 35mm, and 28mm lenses. If you wear eyeglasses, it is a glasses scratcher for me because I push my eye close to the steel-rimmed finder to get that magnificent 1:1 lifesized framing with 35mm and even 28mm lenses (!) by shooting the 35mm framelines as loose guides. That finder is huge and incredible. Its full-frame view is 25mm. Lifesize. The S3 finders tend to be the brightest of the lot, a little flare prone, but not as important with 1:1 viewing. The S3 has a couple of small markings to to help with parallax framing up close. The drawback of the S3 is that framelines are etched fo 35mm, 50mm and 105mm lenses. So, when shooting a 50mm, you still have the 105mm rounded frame corners super-imposed. I found that I quickly got used to this and that you can actually use the 105mm corners to help with rule-of-thirds framing.
The S3 and SP are identical cameras except for the finder. I second the comment above that for whatever reason, the S3s tend to be the smoothest of the bunch. I have an S2, an original S3, S3-2000, original SP and SP-2005. Both S3s are the smoothest of the lot.
I used the S3 for a quite a lot of news photography in the 1990s. My shooting is now more casual and family oriented. I tend to carry the SP because of it's ability to shoot 28mms ande 85mms, my two favorite focal lengths, without unduly scratching my glasses when shooting the wides.
The S2 was my first Nikon RF. I enjoyed it, but it turned out to be a stepping stone to the S3, which is a camera I fell in love with.
