As Sizzers has stated i went from 3 faulty GT30 plasma tvs to a 46HX853 just a few months ago and the reasons i didn't go for a GT50 were many...i would have faced a wait for stock to arrive in the UK as the GT50 range was incredibly difficult to source at that time, i really wanted to make the jump to a larger screen size, i had auditioned the 46HX853 which had an incerdible image for an LED tv and i got a good deal on the HX853 to name but a few.
The review for the 46HX853 had appeared in WHF just weeks before and so i was aware i could get a little extra screen size for a minimum outlay, about £150 after my refund if i recall correctly and so my head was turned in this direction but i didn't just dive in as i went out and auditioned the ST50, GT50, VT50, HX853 and HX753 tvs before i made a decision.
When auditioning i was struck by how close the HX853 image got to a plasma tv as i had never seen an LED tv that even got close up until that point, it seemed extremely natural and not overblown with contrast like many LED tvs and motion was simply exceptional. As strapped for cash points out the black levels don't go quite as deep as on the Panny plasma but what i would say is that at the levels stated it's debatable just how much of that will be truly noticable, coming from a GT30 i certainly don't feel short changed.
The counter argument here is that the HX853 deals with contrast and motion better than the 2012 panny plasma range but again it's a close call except for motion where i found the plasmas juddery in comparison, at the end of the day there is no substitute for your own eyes and it's vital that you get out there and audition them for yourself. Be aware of the potential pit falls of each tech ie image retention, screen burn (although less of an issue these days) and a continuing 50hz bug on the Panny plasmas and backlight inconsistencies or clouding, DSE effect and some reports of banding on the Sony HX853 and decide which works best for you.
I must add that i remain a fan of the image a plasma tv serves up and it remains a natural and enjoyable image but for my own personal circumstances i preferred to swap to an LED HX853 this time and not just because i would have to wait for a larger GT50 plasma or indeed stump up more cash. For me one tvs strength was the others weakness so it just comes down to personal preference and your own usage, for example, when gaming on my GT30 i had image retention issues which i wanted to avoid in my new tv and wasn't convinced a GT50 would solve this issue. That wasn't the only issue of course but in order to stop this post going on for much longer i won't list the rest, in the end the HX853 once calibrated serves up one hell of an image and one i remain happy with now...i made the right choice for me personally and that's what i recommend you do as the tvs are so close in performance terms it's hard to split them.
Get out there and look at them for yourself, at the end of the day you are going to be looking at it for some time to come so it pays to research all the options first, it could even come down to which tv is better styled or indeed the price for example as they are that close in performance and above all take your time to weigh up all the options for your particular needs.
I'm sure either tv will be a wise purchase in any case and just hope that i've been of some help and not confused you further