Hmmm, a fight. Popcorn v Maltesers.
I went with roudcube primarily for aesthetic reasons. Squirrel is no-nonsense bare-bones but I had a feeling that the few webmail users I'm supporting would prefer something more modern looking. Feedback suggests that's been very much the case. Roundcube is also more powerful - it should be at least another two months before users break the folder-filtering.
Gotcha wise, this isn't likely to affect you but I messed up post legacy migration and couldn't purge/re-install a default working roundcube installation. The fix was ridiculously easy in the end -- delete the roundcube database user (as well as dropping the database).
I was also caught on the hop by how quickly users created address books in squirrel and had to google for conversion scripts to import the contacts in roundcube. I've not checked, but at the time I was wondering whether symbiosis backs up the webmail config and data. Without looking, I'll have to assume not.
[Btw, the corrrect answer is Maltesers]