^^^ Interesting that you say that. My impression is that, in general, the vast majority of recommendations on the site are for cardioids, with a handful of omni, hyper or wide-card suggestions sprinkled in...
Which one is better depends a lot on the situations in which you're taping, in my opinion. Like Chris said, it's good to have both...For the bands/rooms I usually record in, omnis are great, but I have a pair of cards that I also use for some venues and artists. If I could only have one, I would take the omnis as they are the right choice for me about 90% of the time. I think for most of the members here, though, that's probably reversed (cards are better 90% of the time).
In any event, for the OP's intended use (taping his own band's performances), omnis might be a good choice because he can presumably go on-stage...
It is true, I think, that in the subminiature mic category, omnis tend to be better performing overall - in that they have a more normal frequency response - than small cardiods.
I think that is generally true with larger mics too, particularly in the low-end...