In principle they are different shells, with different features. There are LOTS of different shells out there, more or less common on different unix variants, e.g. csh, ksh, ash, tcsh and many others. It's quite possible that under your linux system they are actually identical, on my Fedora install I'm on now that is the case:
[chris@thinkpad ~]$ ls -l /bin/*sh
-rwxr-xr-x 1 root root 943360 May 21 18:32 /bin/bash
lrwxrwxrwx. 1 root root 4 Aug 19 20:49 /bin/sh -> bash
So it doesn't matter which one you choose, but then again you shouldn't need to actually make a choice at all anyway, you should never generally need to give any consideration to the shell you use, just leave it alone is probably the best answer. You can see the other shells used with other accounts in /etc/passwd among other places.