I have dived Cat Island and was not at all impressed.  Charming little
island and not touristy at all, as I recall.  That is why I found it
charming.  Would not go out of my way to dive it again.

Have not dived San Salvador but have heard more than once that it has the
best reef diving in the Bahamas.  I say give it a try and let us know what
you think. :)

Dive safe,

/Don.