Tough call actually. Intuitively, I like omni's at the stage lip or on-stage because you like to pick up what's happening on the stage and not be limited by the directionality of the mic capsules. However, if you feel that the ambience of the instruments from the stage will be drowned out by the volume of the gigantic PAs, then I'm not sure going with the omni's would add much presence because the PA volume would be predominant. I've run stage lip before when the PA volume is really loud and my recording sounded like crap because it sounded like my mics were distant (which they weren't since the stage volume was low and the PAs were the major sound source that particular setup). In that case, I might opt to back up a little bit in the crowd, go with cards and point them at the PAs. When you run omni's you always run the risk of picking up more crowd noise from behind, so you also have to take that into consideration regarding whether the volume of the PA speakers will predominate. My point is that if you aren't gonna be picking up much stage presence with the omni's because of the volume from the PAs, then you might want directional mics to pick up the PA and cut down on the crowd. However, if you expect that the volume from the stage is good and won't be drowned out by the giant PA speakers, then DEFINITELY go with the omni's and stand at the stage lip.