Even though mathematically, switching cups is the best choice (They didn't take away the other cup, so therefore it has a higher chance, 2/3 instead of 1/3) personally I would still stick with my initial guess, just because I go with my gut usually.

There's a theory/popular mathematical equation about this out there actually but I can't recall the name of it. :c

(However I voted on the wrong one and voted what I would do instead of the one more likely, whoops.)

Ah here we go :

http://en.wikipedia.org/wiki/Monty_Hall_problem