@mail and horde are totally different beasts. @mail allows you to offer virtual email addresses to your users, whereas horde is simply a program allowing you to access physical mailboxes.
Having said that - I switched to RoundCube for my clients, and they love it.
We're considering that one as well - did you just install it under the /usr/share/psa-horder directory and reconfigure the various files to move to the directory automatically, or something more involved.
Would be nice to know how you set this one up, and whether you set it as an option or by default.