omni caps won't really matter a whole lot. you might pickup more vocals if you are in front of the monitors. if you are still behind the monitors, you'llonly be getting reflections of the original vocal sound coming out of the PA and monitors. it's all about posititioning from the sound source. on stage, with the right config, the instruments will sound very real and it will be a nice reproduction of what happened on stage.
however if you are recording a band with vocals and they are down in the mix to the point where it is hard to understand what is being sung, then what's the point, ya know? (or at least in my opinion)
try moving back to where it sounds best from the audience. decide whether or not the tradeoff is worth it to you. For me, the clarity of the vocals and the handling of the low end are my two biggest critiques in any audience recording.