Which e-mail client is preferable: Thunderbird or Opera?

I have two separate sides to my business and I want to remove them completely from one another. At the same time I would like to try out something other than Microsoft Outlook. So I would like some opinions about whether Opera or Thunderbird is better. Or if there is another I have overlooked? (One thing I don’t want to live without is setting mails to send in the future though…)