I use a SM57a (beta). I like it, personally. I've used the 58beta as well, but sounded a bit thinner to me, but since it's omni-directional and there was more than desirable room noise, I think it was affected somewhat.
If you want, I've got a thread going in the MP3 clinic where the electrics were done with the 57a straight. There's no EQ and just a bit of verb. I think I paid about $120 for the mic at the local retailer. I'm sure they can be had for much cheaper.
As far as the choice between those two mics, I don't know the Marshall, but I don't care for 58's on guitar amps. Just my opinion.