Since this is a US based forum pretty much nobody will be able to form a cohesive opinion since the GT60 isn't sold in North America, but the one thing that stands out in my mind is that the 50 series has gotten tons of complaints that it has a softer picture and lacks detail compared to previous models (i'm one of those complainers and it's valid), but lots and lots of new 60 series model owners here have reported that their new 60 series has a sharper/crisper image than the 50 series models do. And the 60 series is brighter too and has slightly better black levels.
The bottom line is the 60 series outperforms the 50 series in every category. The only reason i can come up with to choose a 50 series over a 60 series would be if you wanted to get the 50 series for a super low price and are willing to put up with the inferior picture quality.
Have you tried the UK based AVForums? They discuss the GT60 there....http://www.avforums.com/forums/plasma-tvs/