I switched to thunderbird about 6-12 months ago since I got fed up with certain issues, chief among which were:
* issues with syncing between google calendar and evolution calendar
* non-persistent calendar view -- back in the days you had the default calendar view, and that was that. You could change it temporarily, but it wouldn't persist between switching between mail and calendar.
Otherwise I've always found evolution 'prettier' than thunderbird, which gives me Eudora flashbacks, and works more seamlessly with PGP (thunderbird requires enigmail which works fine -- but it's still a plugin).
The only issue I've found so far was that I had to close and restart Evolution after adding my google calendar details before it would prompt me for the password and actually sync.
What I've noticed now:
* Calendar view (i.e. day/week/month) is persistent and remembers your preferences
* Sync between google and your local calendar works fine -- I had the appointment on my phone within a minute of entering it in Evolution
* The interface is cleaner, prettier (not sure whether anything has suffered from the deprecation syndrome most gnome-related software has been subject to recently)
* PGP required no set-up whatsoever -- an encrypted email was automatically decrypted without any set up at all. I'm not actually completely happy about that -- I'd prefer that to have been prompted for the password to unlock my key (I'd unlocked it for gajim a few hours earlier, and I guess it remained unlocked)
So while thunderbird has stood me well, I've historically been using Evolution since I switched to Linux (before that I used thunderbird on windows, and before that Pegasus Mail and Eudora) so I'm likely to switch back now that most of the things I were hung up on last time seem to have been fixed.
On the other hand, I'm noticing that I have to close and restart evolution any time I change account settings, which doesn't seem right, and which is fairly annoying.