Both SM57 and 58 are dynamic mics that are famous for capturing vocal performances. They're an absolute go to for live sound, and great for recording all aspects(vocals, drums, guitar, bass, ect) within BANDS. Let's not pretend great vocal performances that surpass alot of vocals captured with condensers don't exist thanks to these mics. If you came up in the last decade, you may have been taught differently, but dynamic mics have always played a role in recording all types of audio. We now live in a world where most recordings are done thru 1 channel on an interface in an enclosed environment. While there are better options for these environments, nothing Salem said didn't hold truth. Depending on the OP's environment, an
SM57/58 may be exactly what he needs.
http://www.gearslutz.com/board/low-end-theory/154745-yes-sm57s-sm58s-studio-mics.html 2 seconds to google search "SM57
SM58 Studio Recording"
Back to the original question, AT2020 and MXL 990 are the same class of mic. Can't say one is going to be "better" that's left to opinion, you won't hear a significant difference between the 2. By "significant", I mean if all you have to compare is the MXL990 and AT2020 they may sound like night and day to you, but the second you add something better, both will sound like exactly 3:21am on a monday when you stagger home drunk to realize you gotta work in 2 hours, lol. The MXL990 should be a good enough learning tool while saving up to make an investment with a higher end mic. Great time to start playing with EQ, figure out good environments for recording, ways to clean stuff up(noise gates, filters, compression/leveling, ect)so when you make a big jump, the knowledge will be there and you sound will be at the most professional level it can be.
EDIT: My bad, thought you were wanting an AT2020. 4040? Why not? Go for it. Decent mic.