IMO, Velvia 50 is much nicer than Velvia 100F (I see both are still offered). Personally I haven't tried Velvia 100 (no "F"), it was introduced since I largely converted to digital.
There are actually quite a few good slide films, so long as you don't go above ISO 200. Film was really at it's peak, best it had ever been, just about the time the "digital revolution" stampeded over it.
If you want higher speed film, I suggest some pro grade color neg film instead. It's less grainy and contrasty than high speed slide (reversal) films. Slide film will usually need to be mailed away for processing, unless you happen to live in an area where there's a lab that handles it. C41 processed color neg film can more likely be processed locally.
If interested in B&W, some of the chromogenic C41 B&W films are useful for local, 1 hour processing... And perhaps more importantly scan pretty darned well if you want to work with the images digitally.
Traditional, silver-based B&W films are richer and can be quite beautiful, but limit your processing options and just don't scan particularly well (largely because the silver halide particles actually block light, so scans get contrasty and lose detail, particularly in the highlights). The best way to convert a traditional B&W film image to digital is to make a print through the normal silver/chemical enlargement process, and then scan that. I love Fuji Neopan Acros 100 and some day might be tempted to umpack and set up my enlarger to use it again.
I agree that medium format, and for that matter large format, are wonderful. But your choice of film and processing will be quite a bit more limited. You'll also need lenses and perhaps other accessories. If you get an EOS film camera, you can use your existing lenses and perhaps most accessories you might have.