What differences are there between the various version of emacs available?
Solution 1:
emacs-snapshot is a development version of Emacs. It has more goodies but likely also more bugs, so use it at your own risk. The others are released versions; the only reason I can think not to install the latest one is if you're very short on disk space.
The emacs package depends on emacs23 (you can see the dependencies in any package administration tool).
Gtk programs don't use X resources (.Xdefaults). Non-gtk (but X) versions of Emacs do.