My Main Rig:
☺1998 Gibson Les Paul Studio DC
☺Reverend Rocco(USA)
☺Martin DM
☺Peavey ValveKing 112
☺Dunlop Tortex(1.35Mm) Picks
☺Everly Rockers - .10guage.
☺Martin - .11guage.
☺Too many effects to list.
I've never used the MXL so i can't really offer any feedback on it, however i would say that both the shure's are very good for recording. I would personally get the 58. Also remember that the recording techniques used are jst as important as the mic you buy
Professional Mixing available at request.

Everton FC
In this case I would have to recommend the MXL's as it is rather difficult to get a good acoustic guitar sound using a Shure SM57. The condenser should handle vocals quite deftly as well. If you wanted the mic for guitar cabs and vocals i would go with the Shure SM57, but seeing as you want to record acoustic guitar I will have to stick by the MXL's.
PM Me for any help you need with recording systems/tips
Quote by BrianApocalypse
Good call

Man, you should be a mod, you know everything.

I'd use the MXL 990 if its for both guitar and vocals. Also, I would not recommend the onyx satellite as there have reportedly been a lot of compatibility issues with windows, don't know about macs.
yea get a m audio mobile pre.

it has some preamps in it.
its a good choice.

and phantom power for the mxl set i say you get.
Fender H Stratocaster USA.
SX P90 Mahogany Hawk
Gibson Les Paul Studio Double Cut

Delta Labs Tube Driver
Hardwire SC-2 Valve Distortion
Delta Labs Stereo Chorus
TC Nova Repeater
Korg Pitchblack Tuner Ped

Fender Super Champ XD
stay away from the 58/57 for the acoustic... it works but better off getting the mxl pack and stereo micing the acoustic guitar (point the Large diaphragm condensor (the big one, also known as LDC) at the soundhole and the small condensor mic (small one aka SDC) at the fretboard and pan them slightly left and right for a KILLER sound)