I don't have any experience with Zix so I can't make any comparisons. However, I can give you my thoughts on Exchange Online message encryption. It's pretty cool that you can send an encrypted e-mail just by using a keyword in the subject. At least, that's how our transport rule is configured. On the other hand, while the sent e-mail is encrypted, the copy that's stored in your Sent Items is not. Anyone gaining access to your Sent Items can clearly see the "encrypted" e-mail's contents.
The workaround for that is to send yourself the encrypted e-mail, open that, and then forward it to the intended recipient. Not pretty but it works.
I think, that if you're happy with Zix, you'd be better off staying with them....at least until the Exchange Online version of encryption matures.
That's my 2 cents.
