Coming from a Leica background and having gotten a new Nikon RF for basically the same reason you are looking at a Leica I think. I can say the New S3 is a great camera. Leicas are still easier for me to use (part being just more familiar) and will be the film camera I'll always have, but if I was going to recommend a RF the new Nikon with the new lenses are hard to beat.
Depending on the money you have to play with, either...
1) expand your Nikon RF system with a 28 or 35, another camera, maybe an S2 and make it simple - one classic style RF system.
2) Get the Leica you can afford, any will do. I'd look for a good working condition M2 (CLA will set you back $200-300 if needed) and 35mm lens (too many aging M3s have finder problems). The M2 .72 finder is just great with a 35. Get yourself Leica lens too - the whole experience - an older Summicron 3rd version. If you want to spend more find a good user M6 for its meter, same basic finder. Don't worry too much about the purchase, you're just trying and if you like it will do a lot more thinking later, and if you decide to sell you'll probably get most of your investment back.
Have fun!