I am wanting to record voice + mic’d up guitar amp via a MacbookPro and am looking at either a USB mic or an interface that can take SM57’s etc.
Whats the easiest way to do this?
I'd go with the PreSonus AudioBox USB. Actually, this one is currently on my wishlist. It's a 2x2 device with dual-purpose mic/instrument inputs (XLR & 1/4"), individual channel trim controls, zero-latency mixer, MIDI in/out, 48V phantom power for mics, and more. It also comes with the PreSonus Studio Artist 2 software for both Mac and Windows so your DAW is all set. It's a pretty good deal for about $150. They also have their bigger Studio series devices which start around $200.
Looks like PreSonus is the brand to go for. I was looking at Firewire interfaces (which they also do) As it can daisy chain with my drives etc. And PreSonus look like the only ones that keep current with OS X 10.8 +10.9 compatibility.
It's good to have some feedback to confirm my thinking. USB Mic's are not that common for Mac (low /midrange ones at least). Don't think i'll bother with them.
It's a bit difficult to advise you without knowing what sort of budget you have, but a two channel USB audio interface is probably your best bet.
These range from the very affordable Behringer UCA202 U (around $20), which only has RCA, optical and phones IO, to teh PreSonus AudioBox Studio (about ten times the price), which has Neutrik connectors (so you can plug your SM57 into one, and a guitar lead into the other). There are other audio interfaces with more channels and/or connectors, built-in mixers as well as other digital interfaces (Firewire and presumably Thunderbolt, though I haven't seen any of the latter yet), but from your question, it sounds like this is the range you should be looking at.
For more in-depth information, you may want to check Sound on Sound magazine (advice given to my by a recording engineer!).