I'd have everything on the SSD.
It will be much faster then any other drive you have, unless you have multiple SSD's.

The advantage of Symlinks is different wow configurations.
By this I mean video options etc.
Addons too, but you can easily enable or disable them on a per character basis, so this isn't much of a gain.
Running 149 clients from a single install (if your comp can handle it) isn't faster or slower then from 149 SymLinks.
The advantage is different video/sound options.