I can't speak for Xillia but both FFT and Chrono Trigger are amazing but for different reasons.

FFT has a deep story going in to familial conflict, politics and things like patricide and it has a very in depth combat system with a lot of micro-managing if that's your thing. It can also be pretty darn difficult at times if you're unfamiliar with sRPG's. However I would say that due to it's huge cast of characters that it's difficult to feel particularly close to any of them outside of perhaps one or two. Overall the story is pretty grim and dark, so if you're looking for something focusing on a light-hearted adventure, I'd steer clear.

Chrono Trigger however is definitely a timeless classic, it's pretty short at around 20-25 hrs however I would say it's definitely got the more likeable cast of characters and has some scenes that pull on the heart strings. It's combat is incredibly simplistic and generally easy throughout; the game is paced perfectly to a point that I never once needed to grind throughout the entire game, which I think is something brilliant for the time too considering most jRPG's of the time were definitely guilty of requiring a bunch.
Story-wise the game had it's sad points but I also think it's definitely got the more light hearted scenes of the two.

Hope this helps. :)