[X4U] Mail.app vs open source mail?

[net kat]

I'm considering migrating from Mail.app to an open source app, such  
as thunderbird, or mozilla mail.

Important factors to me are: ability to set up useful filters and  
folders, ease of migration from Mail.app to whichever open source  
choice looks best, and whether that open source choice on Mac creates  
files which would be useable in a linux or windows environment.

Can anyone shed light on these questions?

thanks!

nk