THX is not the best, it's actually limiting especially if you're going to get it calibrated. And if you want to spend the extra $$ on calibration then the ST30 would be the way to go since you're best image won't be on THX on the GT30.
Through Amazon, right now, no tax & no delivery charge, 30 day free returns & returns are super easy- for whatever reason, 24 month interest free financing. (I bet prices drop a week or two before and/or after Christmas)
60" is enough if you watch cable or satellite. If you only watch a real HD signal then 65" for sure. Consider that the bigger the screen the more increased poor signal quality is and that HD from some sources are really not all that and can produce a really looking horrid image during dark scenes.