I've had real world experience with both, and they each have their own benefits. Personally, I've leaned toward Zimbra quite a bit.
Gmail: Remotely hosted, generous storage, free (assuming you're using the free version - although I don't know if you can with 230 users or not), don't have to worry about your own servers/hardware, they have a great spam filter.
Zimbra: You run your own environment, great web user interface, Zimlets, you can do some cool things with the API, each version comes with new features - I haven't seen a new feature in Gmail for quite a while.
You can get the best of both worlds if you host your mailboxes with a Zimbra Hosting Provider (see the
Zimbra Partners page). There may also be other providers you can find on Google that aren't Zimbra Partners (such as myself, I also host on the Open Source edition for much cheaper pricing, but also guaranteeing uptime and recovery windows in the case of a disaster).
The best feature of Zimbra for companies is the GAL and the Calendar, and the ability to quickly and easily collaborate (share folders, etc.) and work together.
So I guess you just have to weigh out the pro's and con's of each, see what's more important to you, and base it on that. The best of both of them is that they're not Exchange or Scalix
