All the below assumes that emacs is not already running with a server: ie emacsclient has nothing to connect to.
If emacs is not already running I think emacsclient will just try to start emacs, which it will probably find in /usr/bin/emacs. If you have more than one version of emacs install on your system /usr/bin/emacs will be a symlink to one of them. That link is controlled by the "alternatives" system. To change it run:
sudo update-alternatives --config emacs
and select the version of emacs you want.
If you don't want to use update alternatives (for example becuase there are other users who expect to get emacs22 when they run emacs), you can pass a flag to emacsclient to specify which emacs to use if there is not already a emacs running to connect to.
In mut set:
editor="emacsclient -a /usr/bin/emacs-snapshot-gtk"
or whatever the executable is.
PS. I too use mutt and emacs. However I find that using a simple terminal based "emacs like" editor such as jed or zile to be better options as editors for mutt.