The difference you experience between the MacBook Pro (with Emacs version 22.3.1) and the iMac (with Emacs version 24.3.1) is because you have two different versions of Emacs, and the third-party packages and customization files are different. The fact that you are using file paths such as /usr/share/emacs/site-lisp is indicative that you have not used a self-contained Emacs installation. If you are going to be using Emacs on multiple computers and you would like to have the same experience, then you should switch to a self-contained Emacs version with all of the stock source files packaged within the Emacs application -- instead of scattered across your hard drive. The following link will provide you with one such packaged application:
http://emacsformacosx.com/
That self-contained Emacs application should be installed on every computer you are using.
Install your additional third-party packages (e.g., AUCTeX and YASnippet) through M-x list packages, which will put everything into the ~/.emacs.d folder. You can develop just one .init.el file and place it inside that folder also and just copy that one file to all of the other computers.
Options for synchronization of the ~/.emacs.d folder include, but are not limited to, creating a symbolic link with Dropbox and have all your computers synchronize over the internet; or, use ForkLift2 to synchronize files / folders between computers. If you are synchronizing, then you would only need to install the third-party packages on one computer and they will be disbursed to the other computers. Revisions to your .init.el file would likewise be synchronized to all computers.
I would recommend not using Homebrew and Macports for the Emacs installation because they will not give you a self-contained Emacs application. For a variety of reasons that occur during the build process, Aquamacs is extremely complicated to obtain complete control over file locations and thus is not an ideal candidate for purposes of synchronizing with multiple computers.
FYI: I presently maintain identical Emacs installations on three (3) different Apple computers and it is a real treat to have the same experience on every computer. I even have a Windows installation of Emacs on Parallels that uses the same third-party package directory as the native OSX installation of Emacs.
elpaandmarmaladewill default to the~/.emacs.dfolder -- there are a few third-party Emacs distributions built for OSX that include additional third-party packages -- e.g., Aquamacs, Vincent Goulet, etc. For purposes of having identical installations on multiple computers, my personal preference is to have a stock build (--with-ns, but nothing else added) -- e.g., emacsformacosx.com – lawlist Oct 16 '13 at 22:31