My Pentax 645N with the 35mm f/3.5 has in the last 9 mos become my new favorite walking around camera-- for scenics, that is. This despite owning everything from a small 35mm system camera (Pentax LX) to a couple of Nikon F5s and even a 4x5 field camera as well as a couple of digital bodies. The 645N matrix metering is among the best of any camera, ever. The FTZ 500AF off-the-film plane TTL metering electronic flashes are quite good, though they don't quite have the versatility or rapid recycling of the F5/SB-800 combo (and the pokey 1/60 shutter sync speed renders outdoor flash pretty useless with modern ISO 160 or 100 films). Don't own a P645 leaf shutter lens simply because these don't enable TTL fill-flash.
Having shot performances in venues like you describe, however, I'd have to say that medium format would be pretty far down the list of format choices, for me. Sticking strictly with film (though for such PJ work, most won't be shooting film these days for the low-light ISO improvements, the traditional argument in favor of medium format at high ISOs in enlargements versus 35mm at high ISOs, effectively no longer exists. ), MF loses out here to modern 35mm gear for several additional reasons. First is that unless the performer is seated in a chair or standing in one spot, it's going to be challenging to nail focus, not to mention freezing decisive moments with medium format SLR gear, mostly because of the long lag after pressing the shutter release. (IOW, when the mirror is swinging up out of the way. With my P645N this lag is along the lines of 1/4s or 250 milliseconds whereas my F5 is less than 1/20s --42 ms, if memory serves). Too, AF in 35 is more sophisticated and the lenses faster. And many 35mm systems have simple wireless slaving options, some even TTL like the F5.
Not that you can't make 2-1/4 or 645 film work in this situation, e.g. scale focusing and blast everything with 400 watt-seconds of lighting-- whenever possible. Keep in mind though that many if not the majority of performance venues have prohibitions against flash.