I think this is one of those "how long is a piece of string" questions.
I own a Hexar RF, a Contax G2, a Leica M (an M3), a Hexar AF and both manual and AF SLRs. If this were me and I needed to be sure of catching key shots in low light with critical focus accuracy, on film, using available light then I'd choose either a Hexar RF or an autofocus SLR. The SLR (if there's on-camera or on-non-firing-flash focus assist) would probably work better in really low light (note: I can't focus an MF SLR to save myself in low light). But the RF would work better if there were enough light to get decent contrast on the RF patch.
But that's just me (and on those assumptions).
My problem with the Contax G2 and Hexar AF, for catching shots that have to be caught is the lack of confirmation of the point of focus. Assuming correct alignment, a coincident rangefinder will do that with precision. The ground glass on the SLR will do that with a deal less precision.
With the AF on either the Contax Gs or the Hexar AF you just have to trust it. Now it may be that I've owned neither camera long enough to be fully around all their quirks, but I've found that focus is precise only mostly. My fault, not that of the camera, but (so far) I've found the dividing line between a hit and a miss is pretty fine and I don't always get it right. And there's no in-viewfinder indication that I've got it wrong.
My reason for prefering the Hexar RF over a Leica M comes down to two things: film handling and auto-exposure. I find the .6x finder on the Hexar RF is good enough to consistently focus a 55/1.2 and 75/1.4 accurately and fairly close up, which is good enough for me. I like the ability to see 28mm framelines easily enough while wearing glasses. I think that precise, stepless, auto-exposure might be pretty important for nailing exposure for weddings (notoriously tough, though I have no real personal experience). And I imagine you'd go through a lot of film at a wedding, so I imagine film handling would be at a premium.
That having been said, I imagine a Leica M7 (or M8 if you want digital and have cash to burn) would solve the AE problem (the G2 and Hexar AF both have excellent AE, just by the way). The M8 obviously wouldn't have a film handling issue, though the M7 would. Either of those would be quieter than the Hexar RF (though I don't find it especially loud, and its lots quieter than almost any SLR).
A ZI body would solve the AE and film handling problem (not quite as nicely as the G2, or either Hexar, but it should still be pretty good). Its also said to have a truly excellent finder and I imagine it is quieter than a Hexar RF but louder than an M7 (and all are hugely louder than a Hexar AF, even when that's in non-silent mode).
And that's why I say its a "piece of string" problem. There are so many options, and so many trade-offs, that it comes down to what you want and what trade-offs best suit you, and your style of working.
...Mike