The difference is that one is a condenser and the other is not.
The MXL may be a better choice just because it is a little closer to that "classic studio sound" on the other hand, SM58s are really good mics overall...
The MXL will probably pick up more, including mess ups, just being a condenser and all... REMEMBER you will need phantom power.
I have had lots of luck with MXL 4000 on vocals (here is a song I recorded the vocals using
the MXL 4000 http://www.electricblustudio.com/St...8/11/22_I_Hear_A_Voice..._Monty_Mcintire.html) and they may be similar mics... Don't forget, I was using a good Pre-Amp (A UA SOLO 610)
As for the SM58, I know Led Zeppelin did lots of their recordings with one and it came out great didn't it?
Hope this Helps!
-Julien
The Electric Blu Studio
www.electricblustudio.com