Impossible to make the onstage config call without knowing the venue, the band setup, the audience, the PA..
Any particular reasons for those two choices specifically?
15' is a lot. Not saying it wouldn't work but 3' or 4' apart on stage would be a safer bet in most cases and 6' probably as far as I would go before I'd want another mic in the middle. Either way, you're going to get more audience and room with omnis. That might be good or bad.
Going with the omnis on stage, and not knowing the other details, I'd space them 3' and run the cards PAS from were you describe near at the board. That may be the safest, relatively conservative choice of all following..
110* X/Y cards might work, 120* might be better. Nothing wrong with either. Although if I was only using a pair of cards onstage and nothing else, I'd probably use something similar to ORTF. I'm just not much of an fan of X/Y for this kind of thing, unless it's mixed with the spaced omnis in which case it can blend nicely. X/Y in the center with omnis flanking wide on either side can work very well. But since you probably want your other pair out in the room to get PA, you won't be using all four mics together on stage that way. Also in this case, you probably don't want the omnis out in the room, with perhaps two interesting exceptions:
1) If you can setup wide omnis so they are sort of stack taping the PA, the PA sound they pickup will actually be quite SBD like, and the room and audience reaction sound they'll also pickup will be nice and wide and decorrelated. I'd probably choose an ORTF-ish setup onstage in combination with that though. You'll have plenty of 'center' from the SBD-like mono PA. That could work really well, especially for any vocals, even if in this case that's just banter and introductions.
You might have heard an example of two omnis setup that way (combined with no other mics) a number of years ago after TS user Illconditioned posted about permanently installing a wide spaced hanging omni pair something like 4 to 8' or so (I think, can't remember) in front of the hanging PA speakers above the crowd, and posted links here to recordings hosted at his website. When I listened to some of those, it to me sounded closer to a good stack-tape or straight SBD during the music, while between songs it sounded like wide spaced omnis on the audience applause, talking and room. An on-stage pair would be a perfect addition to that, with great potential to improve the more straight-SBD sounding music parts, through the addition of things the on-stage mics do best- dynamic instrumentalist transient attack clarity from stuff like drums, nice rich tone from the player's amp cabinets, nice 3d depth from the rich on-stage early reflections, left-right placement stereo imaging, all the gravy, baby.
2) Wide omnis boundary-mounted against hard, flat backwall can work nicely and sometimes even get a cleaner PA sound than cards from way in back. But unless they are right up against the wall, I wouldn't use the omnis more than 1/4 way back from the stage. X/Y cards on stage could a good choice with that though, with it's solid, drier center.