Some thoughts...
In general Unix Admins get more money than NT admins.
Unix is not going away any time soon (even though MS is trying hard to make it happen).
NT is going away...to XP or other things, so skills will under go serious changes in the near/mid term.
Big Iron is much more likely to continue to move to unix solutions and not Windoze. Big Iron is also experiencing a comeback since it is proving so effective in handling internet loads. Big Iron also pays big.
Unix isn't for beginners, which translates to less competition for jobs.
Unix is less prone to security problems which can mean less possibilities of getting your tail in the ringer for missing something.
It is definity your choice. But my advice is that if you are intelligent and capable give Unix a try. You will probably do very well. If however, you tend to be lazy or don't capture ideas very well then stay with NT because you won't do well on the Unix side.
This is not meant to insult, it is merely based on the fact that MS places so much emphasis on ease of use that a lot of people that would never cut it on Unix are considered acceptable in the NT world. An honest self evaluation here could save you a lot of grief.
My advice: If you think you can cut it go with Unix. With a good NT background you really won't have trouble switching back to Windows if a better opportunity comes along. (Again, if you don't think you can cut it, don't screw up your career--stick with what you know!)