I'd have to go with FreeBSD, as far as i know: it has more support / docs out of the BSD family... I really like- although at times can be both frustrating and fun :-) As far as making the switch from M$, make sure to read as much documentation as possible...

Here are some of my fav links:
http://www.freebsd.org
http://www.freebsd.org/doc/en_US.ISO...ook/index.html
http://www.freebsd.org/doc/en_US.ISO...k/mirrors.html

Newbie Links:http://www.freebsd.org/projects/newbies.html
Un*x Help for Users: http://www.mcsr.olemiss.edu/unixhelp/
X Windows User's guide: ftp://sunsite.unc.edu/pub/Linux/docs/LDP/users-guide/
The Complete FreeBSD: http://www.freebsdmall.com/cgi-bin/fm/bsdcomp
Unix Int. Course: http://www-wks.acs.ohio-state.edu/unix_course/unix.html