I think we should make this question a sticky at some point...
To answer the OP, there are two things that you should be concerned with when recording outdoors - 1) Distance to the stage or soundboard and 2) Wind.
Starting with #2. If it's windy where you're recording, you could use your CARDs with Dead Rats (Chris sells these for $30 + shipping ATM). If it's not an open situation, they might be "too noticeable" for the event, and this leaves you with the option of OMNIs (and no Dead Rats).
#1 - everybody's been answering this one already. It depends if you're close to the speakers stacks - then it's OMNIs all the way. Figuring the soundboard is, say, 200 ft from the stage (usually where it is in an Arena or Amphitheatre), then my rule of thumb is cards if you're anywhere from 2/3's to FOB. Again, this is subject to personal taste. And again, if it's windy, see what I just said above.
Enjoy your show...