Tough call..omnis pick up alot more low-end (mud) so.. I dunno..
I've used my omnis only a few times (I prefer cards)..it seems as when you are in the optimum location with a good mix, omnis are the preferred capsule..but when you're in less a less than ideal location/conditions..they arent. I think if I could choose my spot, i would go FOS, at least if youre gonna get a heavy bottom end, being that close to a stack, the proximity to the source would at least drown out most of the crowd chatter (which youll probably pick up alot of the farther back into the venue you go).