I have two SM58s as main vocal mics for acoustic gigs with a good quality PA and they are just as good as you would expect. I also have two cheaper mics (Samson and a Thomann T-bone) which are acceptable but need care with EQ.
But having tried the SM48 I would not buy any other budget mic. It's not as warm-sounding as the 58, which I would always choose first, but as a backup mic it's perfect. It's a bit brighter but the same solid Shure quality. If it was the only mic available I would have no qualms about using it whereas the other cheap ones always feel like second best.
And of course it has the professional look, nobody will be thinking, "hmmm cheap mics."
I just wish there was an SM47