Plesk is actually pretty decent, however I do respect the fact that once you are used to cpanel you like to stay with your accustomed ways.
Personally I think both work, and thus not going to debate that. What I do think is, since this is a opensource webmail thread, you should at least look into Atmail, it really is very appealing, even if you are used to horde, I think Atmail is a newer better alternative to Horde.
Make no mistake, I do support Horde as a second choice, which is what opensource is all about, but don't dismiss Atmail because you don't like Plesk. I've grown quite accustomed to Plesk and find it to be just fine.