Tactics is a lot better than XII, IMHO. FFT sucks you in and is far more addictive, so to speak, than XII.

When I was playing Tactics, I found myself unable to stop playing for an extended period of time. But XII on the other hand just never grabbed my interest to the point where I couldn't stop playing. Tactics, as others have said, has a much better development of the plot, and keeps you interested, as well as simply having a much better plot IMO.

Tactics is easily one of my favorite Final Fantasys and I highly recommend it.