If you're just dipping your toe into stereo photography I'd definitely opt for 35mm rather than 120 as the equipment and film are both cheaper. The Realist is a good starting point as they are not too expensive (for the f3.5 versions) and are built like tanks. If you haven't already done so, have a look at DrT's web pages:
http://www.drt3d.com/3DRealist/ DrT sells Realists that he has serviced - I don't know how much he charges or what your budget is, but If you buy a Realist, unless it's been serviced expect to have to sort out a few niggling faults. They are built very ruggedly, some would say they were over-engineered from a mechanical point of view. The youngest are now approaching 50 yrs so the main issues tend to be with dried out lubrication and sometimes corrosion, usually only cosmetic. If you're of a practical disposition there's little that can't be sorted. Working on Realists is more like working on a car than working on a watch!
I also have a Revere 33, overall more convenient to use but the film transport mechanisms can be troublesome (mine was). They can be fixed but though rugged by today's standards aren't quite as bomb-proof as the Realist. The Kodak is as you'd expect it to be coming from Kodak - a bit more cheap and cheerful but does the job very well. IMHO Kodak got it right in that they didn't make their cameras as durable as the other two, but few consumers that the cameras were aimed at would ever be likely to wear out their Kodak stereos. Incidentally, my Kodak stereo isn't fixed focus, it has the distance engraved on one lens and portrait/group/landscape engraved on the other. Setting the speed and aperture is rather more fiddly on the Kodak than on the other two.
With 35mm you have easy access to B/W shots in a Holmes viewer (if you develop and print yourself) as well as transparencies. Mounting those requires time and patience but is worth every second when you're blown away by the finished result. Another technique I've used is to hand tint B/W stereo prints. Have fun!
Steve