Thunderbird+IMAP is faster to use than Gmail to me. Last time I checked, Gmail was less responsive and seriously lacked keyboard shortcuts -- very mouse-driven.
IMAP offers nearly the same degree of portability as Gmail, the downside being you have to install/configure an IMAP client on all machines. However, I mainly use three machines, and my IMAP provider also provides a webmail interface in case I'm on the go. With Thunderbird I also download my messages for offline, which has been useful on several occasions.
You could use Thunderbird to access Gmail and potentially get the best of both worlds, however I haven't had any good reason to try this -- seems like unnecessary complexity.