agreed on all fronts. It is nice to have the option of cards or omni's. mics with removable caps, or what you have, bring both sets. If you know a band and venue, sometimes you can guage the crowd. For example Ray Lamontagne in a seated show, i was pretty safe in the 5th road Dead center to run some omni's, but had card caps just in case. Avett brothers, didn't even bring em.
IMO omni's will sound better in a "perfect" world situation with a great sounding room and a respectul crowd, they'll win every time.