It depends what sound you want. If you want that “audience taper” sound, then omnis, but unless you’re experienced, it’s hard not to get too much atmosphere, and not enough main instruments. I certainly couldn’t when I recorded myself that way. Ask Dan for advice if this is the direction you choose.
I would place SM57s on the amps, myself. I wouldn’t worry too much about bleed, you’re going to mix it all together in the end.
That said, “both” is the choice of professionals. They like to mix some ambience with the close-microphone recordings for “that live sound”. And you certainly have enough microphones to pull it off.