well, since nobody else has fielded this, here's my limited knowledge from reading the board for awhile.
Cardoid mics pick up sound in a mushroom cap shape (pardon the bad ascii art)
___
( \
mic ) probably more like this =D
(___/
Omni mics pick up in more of a circle and are thus a bit more forgiving if you move when you are stealthing, but they will also pick up more things behind you than the cards...
Anyone else can feel free to chip in here.
I picked up some cards because I heard better things from various people about cards than omnis.
Another Idea is listen to some mic samples and see what mics you like to hear. You're the one that will listen to the tapes, so find some mics that sound good to you.