I got the 46 inch PZ81 yesterday as a replacement for a 50 inch and I sit about 10 feet or so away. Having had it for a day I really couldn't have told you whether it was a 46 or a 50 if I didn't already know! Either will be an excellent choice but you could probably be well on the way to a Blu Ray player with the price difference (if you don't already have one)
going to buy a panasonic 50" plasma tomorrow and i sit about 12 feet away from tv have a 43" plasma now looked at gettin lcd but after hearing probs with sony large screen lcds decided against it, believe me your viewing distance is fine.