They share the same capsule, however, again the differences are due to
the wire basket and foam on the SM58.
If you have someone who REALLY pops their "P's", it can be better to go
with the SM58 over the SM57.
The '57 is clearer than the SM58, this is why they sound different to you.
Objectively, the SM57 is more generally useful, due to the greater
coloration the SM58 has. Simply back off the SM57 to reduce the proximity
effect. Most home recordists, including myself, don't have professional
monitors in an acoustically treated room to better hear the differences BTW.
Chris