MS might work well since you can control the stereo image in post. It will also help reject some of the audience noise, if that's a factor. Omnis probably would sound great, too, but may depend on the audience factor. In either case, don't run too close - you want to give the voices a chance to blend. For my a cappella recordings, I've typically run ~12' - 20' back, depending on the venue and layout. I've also found I far prefer baffled omnis to split omnis. I don't care for the phasing issues I get running split omnis (perhaps I need to close the distance, but in some cases have run as small as 2').