Hi
I have both an SM58 and a Rode NT1.
The SM58 is fine for stage vocals. It's fine for recording vocals (Chilli peppers used one exclusively on 'Californication', not sure about 'By the way'

. It's usable for recording an instrument. I got reasonable levels when recording an acoustic guitar, but the quality was not fantastic. Not got a pad switch on, have you?

I have miked up an electric cabinet with it as well. Again, not really what it's meant for, but it was usable.
The Rode NT1 is more difficult to use on stage. It is better for recording vocals than the SM58 IMO. It is better for recording an acoustic than the SM58 IMO. Good mike for the money. Acoustic recordings were much more transparant with the Rode.
But don't let the shop tell you that an NT1 is better for a weak vocalist. If anything, it is more likely to expose that weakness as it has a more natural sound than the SM58. With the SM58, it is fairly normal to be close enough to eat the thing, thus making use of the proximity effect bass boost. You should be able to get more than enough level out of an SM58 for vocals. Whilst you can sing close to the Rode, you will generally be using it 6-8 inches away and you will need to buy a pop shield or make one with the missus' old hosiery. At least I tell her that's why I want 'em

And you'll have to make sure everything else is silent. The Rodes will pick up a moth at 100 yards
Just be prepared to EQ differently with the Rode, should you buy it. It is a very different beast.