hey i own both sm 57 and 58. and what i can say is that the sm57 requires more gain than the sm 58. though sm 57 is famous for miking drums and amplifier, i somethimes(just for fun) use the sm 58 to record guitars too. the sm 58 (if position closely) can give added bass reponse so it'll be quite interesting if u want to get a more fatter sound.
anyway, thats just my opinion, but wat i can say is, just experiment with which ever mics u have, place them at different angles or combine both together, each at different distance away from the amp. u can get a more ambient(not sure if i spelt it correctly) feel to ur guitars.