The Shure SM58 asnd SM57 are NOT omnidirectional. Nor, as has been said, are they good recording mics and they are designed for close miking anyway.

I'd use my AKG blueline with the omni capsule, but that's because
I already have one. I think it would sound pretty good - I haven't used it yet but as a cardioid they make nice XY stereo recordings and I can use a 12V battery box for phantom power.

You need either a very absorbent studio acoustic or a very good natural sounding acoustic that you can use as part of the recorded sound. It won't be stereo, of course.

Omnis don't have to be expensive - the cheapest mic you can buy is a Panasonic electret capsule and they sound remarkably good for the price are omnidirectional. For about $2 you can hardly go wrong!

Anahata