Personally, after using Outlook Express, Thunderbird and Outlook 2000 and 2003, I started using Gmail with it's web interface a year ago and I just love it. You also get easy access to Google Calendar, Google Docs and more, right through your gmail account.
Before I used Gmail, for a thick client I used Thunderbird at home and Outlook at work. I would say, I liked them both equally and with a few addons I got the same features in Thunderbird as in Outlook. I would go with Thunderbird, though, because it's free and I would have the option of adding more features from it's addon collection.