and to be honest I haven't used TKL Zimbra either so I can't help a lot.

I would imagine that TKL would include many more ways to interact with the underlaying server such as SSH, SFTP, Webmin and Shell-in-a-Box (WebShell). That would make TKL potentially more versatile IMO. TKL is also available in a number of formats eg ISO, VM image etc (not sure how that compares to Zimbra). Have a look at the TKL Core page, all appliances are based on Core. For that matter, the TKL Zimbra page (you already posted the link) may also help you make a comparison. I wouldn't be surprised if the official release was a later version too.

The devs have 64 bit on their (never ending) 'to do' list although the next versions (based on 32 bit Ubuntu Lucid - yet to be released) should hopefully support lots of RAM (theoretically up to 64GB via PAE) but we'll have to wait and see. Don't hold your breath waiting for a 64 bit version.

Is that TKL is already installed. That's the main advantage of using TKL. You don't have to go through the installation process, you get an already-installed-and-configured system. Also, other advantage would be TKLBAM! its going to make your backups (and migration if needed) of the server very easy. 

Can't tell you about the 64bit status. But I understand your question, as Zimbra is deprecating their 32 bits versions. Maybe this is a candidate for a 64bit appliance. I still prefer to run my 32 bit appliances under a 64 bit hypervisor.