I'm running Apple Mail 5.3 and Thunderbird 17.0.4 on Mac OS Lion 10.7.5. The system I'm running those apps it's an iMac with 3Gb of RAM and 2.4 GHz Intel Core 2 Duo.
I run both applications at the same time and I got those results looking with the Activity Monitor.
As you can see, Apple Mails consumes a little bit less than Thunderbird but uses more virtual memory.
My question is, which one has better performance? Apple Mail or Thunderbird? I ask this because that was a puntual test, and I don't know if they get stressed when looking for RSS feeds or email on the inbox.
Do you know any other mail app which works better looking on the performance?