+1` on the XM8500s. I use them, and they're pretty solid. Metal's kind of hard to bang up badly, and for the price, great vocal mics. They do have a bit of a high mid presence that makes them not so great on instruments and amps, but there's other mics for that.
Back to the original question, it depends on the group. Some groups may want handhelds, some may want to gather around a common mic so they can mix themselves. Have enough handhelds for the largest group you'll see, but also provide a large diaphragm condenser setup. The stereo pairs described above work great for recording large ensembles, but barbershop quartets will often be too close for that technique to be effective (you'll have hot and cold spots, etc), especially if this is for reinforcement applications as well. You can play around with gating, but it's VERY difficult to get a gate set on this type of setup so that it's transparent. I much prefer to ride the fader for noise control on this. To do it, you really need an expander, not a gate, and the gated signal should NEVER be sent to a recording.
I personally have (and use for this type of thing) the
Marshall V63M which is a nice sounding mic, but as with any wide pattern condenser is a bit subject to feedback. I have no problems with 3-4 people gathering around a single mic and being heard, and I have used a stereo pair on a stereo adapter, but I find that in this application, splaying works better than crossfiring (or in mic-world, XY configuration).
The other (probably better) option is a single omni. You may want to look at the
Behringer C3 or something similar. I've used the C3 too, it's quite good. This mic is switchable between cardioid, omni, and figure 8, a pair of them could be set up in several ways:
1. Two cardioids, splayed or X/Y configuration.
2. Two omnis, spaced a bit for wide pickup
3.
Mid-side configuration (better for recording) where one is set for Fig-8, one is cardioid, and the F-8 has its lobes to the left and right, and nulls front and back, is fed to two channels, panned hard L and R, with one channel phase reversed. The cardioid then fires straight ahead and is fed L and R. This creates a subtle stereo image with good mono compatibility. I haven't read of a lot of people using this for reinforcement, but it would give you stereo seperation (you could potentially play with phase to help control feedback) and a pretty wide pattern (see the link for details).
Anyway, hope all that helps. Oh, and if you do go with condensers (which you should for this) get windscreens that completely cover the mic's pickup area, front and back, and buy shock mounts if they don't come with. I started out with just regular foamies, but still got plosives where just a millimeter or two of the grill was still visible below the windscreen. They do make larger versions, I have some that completely cover the grill and go down over the body of the mic, and they work great.
Actually, that was the gig that got me started with BFD's, the wedgehorn 10, there were times when I'd go up to the stage and someone would have a jacket stuffed in it 'cause it was too loud. I can't imagine, with only vocals through the pa, I would get readings on my meter at 117-120 almost all night at the booth. Sorry, got a little ot 