I've done a lot of experimenting with really low light with fast lenses and fast film. I have a Hasselblad with a 110mm f/2 lens (I bought this after trying the Mamiya 80/1.9 and the Norita 80/2) and have used lots of Delta 3200 in 120. I've also shot lots of D3200, Tmax 3200 and Fuji 1600 in 35mm (Fuji 1600 is definately slower than the others, but much finer grain and I think it looks the best but won't use it faster than 800).

I find that, in general, I get much better results with 35mm in really low light. It's easy to find f/1.4 lenses in 35mm and even faster lenses are available. You can get fast rangefinder lenses in 35mm that help a lot with slow speed handholdability. Although writing this always seems to spark controversy, depth of field seems much less at f/2 in medium format than at f/1.4 in 35mm. Precise focusing in medium format also seems harder to me, but that may just be me.

Of the 3 fast B&W films, I much prefer the look of Fuji 1600 as long as you don't need to shoot faster than 800 (it has a very short toe and loses shadow and even midtones if you try to push it). I also like Tmax 3200 better than Delta (these choices favor 35mm since Delta is the only choice in 120). I find Delta to have flat highlights and look muddy when the other films sparkle, but I know other people who prefer Delta to the other films. After trying many developers, I get the best looking negatives from all three films in Tmax developer. This is especially true for Delta and the Tmax developer increases the highlight contrast and keeps the Delta muddyness at a minimum. Tmax developer also gives equal or better film speed to any of the others I've tried. I find all the films to look pretty good up to some speed, then kind of fall apart because of lack of shadow/midtone details. Going to a bigger negative helps with grain, but doesn't really increase the usable speed.

I know I'm probably writing blasphemy on this site, but I've actually gotten the best handheld low light results of my life recently using a Digital SLR. I have a Minolta 7D with a 35mm f/1.4 lens. ISO 1600 looks like 400 speed film, 3200 is better than any 3200 film I've used. 3200 is the speed limit, but I've never been satisfied with anything except very special cases trying to push film faster than 3200 anyway. The built-in anti shake allows slower shutter speeds than any rangefinder. The shorter normal lens gives better depth of field, and the Minolta 35/1.4 lens is the best 1.4 lens I've ever used (and always a bit too wide for my taste in 35mm). I much prefer looking at B&W prints I make in the darkroom to anything I've seen from a computer printer and I much prefer using a nice medium format camera with B&W film in general, but I still find the digital SLR to have a strong advantage in low light and find it replacing my 35mm which was only really used for snapshots and low light anyway.