Not to mention that even comparing the two is an insult to Xenogears. The first Xenosaga is actually pretty good, but the second was horrible, and I never made it through it so I never got to play the third one, which I've heard is also good. But even the first one did not come close to Xenogears.

And as already said, the two are completely different in terms of the world, the characters, the storyline... The only similarities are the Xeno- in their names and a few things like the Zohar.