First of all, I don't think you should abandon you Win2K and MSFT expertise. Interoperabiity between UNIX-based systems and MSFT based systems are becoming more and more critical. Your expertise in both areas is very marketable and extremely useful.
As to 'picking one UNIX', if I had to pick one and only one, I would pick HPUX because HPUX is a hybrid UNIX and experts in HPUX get paid A LOT of money !!! Experts on Linux, BSD, etc do not command the salaries of HPUX experts in the US.
Second choice, SUN Microsystems. SUN worshippers also make 'the big bucks'
I started in HPUX and have working with just about every UNIX on the planet. However, I do not bill as a UNIX admin and work high-level architecture and policy for a living (it pays more, much more but is much less rewarding). Personally, I love Slackware/Linux but that love 'don't pay the bills and buy the groceries'
Guess you have to decide if your decision is based on $$$$ considerations or your personal interests. Often, unfortunately, they are not the same. Also running a home business or service is much different than working for a mega corporation.
Perhaps someone else can jump in and advise you as well