Mics and preamps .. always fun to talk about.
Yes, if you look at old pictures of vocalists, you'll see lots of SM-58s. I've had a few vocalists insist they get to do a few tracks using this mic, if they agree to try the one I selected (often LDC).
However, I question how many of the other mics on the same stage are SM-57s.
You won't see a SM-58 on a snare or a guitar amp as often as an SM-57 at the same level of performance. They both have earned their place. If money isn't the issue (as mentioned originally) having both would be nice. I'd rather use a 57 on snare, guitar amp, and acoustic guitar than a 58. For myself, I like my vocals in a 57 more than a 58. But I know many vocalists really like their SM58.

As far as condenser mics, there are lots of good ones, and the preamp being used can also make a difference as to which is better in 'setup A' vs 'setup B'.
I think having a good preamp opens the opportunity to make every mic better, so it can be a wise investment in a different way. You may not 'need' it, but if it makes lots of other equipment better, it can be a valid wise investment.

I notice 90db mentioned a Studio Projects mic; for me I've never regretted getting a Studio Projects preamp.
All my mics sound better through the VTB1, including the 57 and the 58, than when using the same mic through one of the mixer preamps or such.

Just something to consider. I feel I took too long to learn this lesson myself, so thought I'd pass it along as related to the thread.

I do not work here, but the benefits are still awesome
Make your sound your own!