I don't even think that Ubuntu for desktop isn't even that bad. It's not what I would use though on my personal home rig, but it seems to be a solid option in professional server business. I would say, if your clients are happy, then you did a good choice so far.

One of the biggest strength pro Ubuntu is the big community and help you can get from. Mint is probably a good choice too, but I'm not sure if that is a good one for business. Me making fun of Ubuntu is more about the perception from the community and home user, less about professional users. I think especially in business if you are in doubt, recommend Ubuntu.

Edit: Nothing important.