I'd say GameStop, if only because the price + Edge + 20% Off PS2 Coupon (when it shows up) makes it ridiculously cheap (~$13). The only problem, at least through my observations around here over the past 18 months, is that unless you hit a GS that has RPG fans trading in their beloved games, Xenosaga 3 tends to come in pretty beaten up and worse for the wear.
Just a tradeoff, really.