I don't know about 50", but for 60" the only documented difference if 2kg weight. Nobody was able to tell (not LG reps, not sales reps) otherwise, so I was able to get price match at Visions (60PG70) against Future Shop price (60PG60).
Worldwide, there are differences. PG70 has networking abilities, PG60 doesn't. But for Canadian market they removed that feature and just kept two model numbers so each store can deny price matching if they really want. Comapre specs at LG Canada web site and if there are no differences print them and push for same price on PG70 as it is on PG60.