Evolution had all the features I wanted and none of the features I didn't care for. I'm not saying it's perfect but after spending hours trying to get Thunderbird to do what I wanted, I realized it was just too painful to use Thunderbird.
If for example, you wanted PGP support, good message threading support, force plaintext if available, Google Contacts + Calendar sync, script generated signatures, ActiveSync support, mbox spool support, maildir support, and what have you, this is all integrated into Evolution by default. A lot of the addons on Thunderbird that accomplishes these things feel really subpar. I recall at one point accidentally deleting all my contacts on Google via an add-on for Thunderbird. Thankfully, I had my contacts backed up, so I didn't suffer too much from that.
That being said, if all you want, like the majority of people, is a simple mail client that you can use to read mail from, Thunderbird is much leaner and probably better. I feel Evolution more suitable in an enterprise environment due to Activesync support and due to its similarities to Outlook. Since Ubuntu's becoming more and more of a personal desktop OS rather than an enterprise OS like what Red Hat offers, I would say their strategy to adopt Thunderbird was a good one.
Additionally, Evolution used to be a lot slower than it is now. It was practically unusable if you had a high volume inbox. The older version of Evolution is used for the Windows version wchich you can get here: http://www.dipconsultants.com/evolution/, making it easily inferior in the face of Thunderbird on Windows.