I try to remember that I am recording the room. Omnis hear all of the room. Cards hear mostly what is in front of them. So unless you want a lot of the room, I'd go with the cards.
Agreed as to cards. I've recorded a bunch of friend's bands in my living room and have had the best results with cards in coincident X/Y. B/c all the living room recordings I've made are of bluegrass bands, we usually have to mess around with where we put the bass so as not to lose it behind the other instruments crowded around the mics (like an old-timey act). I've also had to train the vocalists to back away from the mics since they're used to screaming into a single mic on a stage in some rowdy bar, and if they don't back up, the vocals (and mandos and banjos) come out way too hot and the guitar and bass are buried in the mix, so the "mix" ends up being all about how loud each instrument is and where it is relative to the mics. I also do everything I can to dampen the room, such as putting a rug down on the wood floor, hanging curtains over the glass windows and doors, and generally trying to baffle the large flat walls. Room treatment makes the biggest difference, imo - more so than mics even.
If you're recording an amplified band you might have an easier time balancing all the instruments in the mix if they're using a PA, but for acoustic bands you should record a track, listen to it, tweak the placement of all the instruments, record another, rinse, repeat...