The best answer I can give you, is you make more money doing what you're GOOD at. If you happen to be better admining Windows than *nix, then you're better off doing Windows because your work will show to your bosses. Same goes for anything else really... if you're better at Unix than Windows, the converse works though the salary CAN cap out higher for Unix than Windows admins. If you like Unix a lot, but devote more time to learning about routing layers and stuff like that, then there's another level to make money.
Possibilities are endless, but don't do something because "it makes a lot of money". You just might suck at it, then where will you be? 🙂