Mozilla Thunderbird vs Microsoft outlook

For those techies out there or if you just have a preference, which e-mail client do you prefer? If you were setting up for a small business as opposed to simple personal use, which would you choose? I know that each one will have positives and negatives (open source vs closed, free vs $$$, no calendar vs calendar, etc...) but overall which would you say is best?

Please vote and feel free to add your opinions.

The only reason anyone outside of corporate networks even uses Outlook is because of Outlook Express bundled with Windows.

For a small business, I don't see any reason to use the commercial version of Outlook at all. If you want simple, use Windows Mail, if you want better, use any of the free clients like Thunderbird.

At my big and now mid-sized corporate jobs, I've used Thunderbird even though the servers are running MS Exchange. If I had to deal with more meetings, I would probably switch to Outlook, but since my meeting volume is minimal it doesn't bother me that much. When I need to send or accept a meeting invite I just use the Outlook web client.

If I were starting a small business, I would want to use Thunderbird but I'm not sure what alternatives to Exchange calendar are out there. I guess you could use Google Calendar, but then there are privacy concerns.

Thunderbird + Lightning. I sync mine to Google calendar so I can use it on my Android phone, but I'm not running any real backoffice servers or anything because I'm solo. If I were going to install something for shared groupware applications, I'd probably go with Zimbra or something similar.