Do you own the 2nd Xenosaga? I didn't think you'd even played the first one. XD

Either way, I'd probably say go for FFXII... depends on how much you enjoyed the first two Xenosagas (and if you haven't played them, I really don't think that being a completist is enough justification to buy it instead of something you'll actually play, but that's just me. XD). I think FFXII has got better reviews in general, though - plus, it's FF! :)