The best thing to do would be to see both sets on display @ best buy.
I had just done that yesterday, and must say that I was not at all impressed with the F8500
I had very high hopes for it, based on all the hype in the forums/pro reviewers.
The magnolia staff were nice enough to give me the remote to play around with the F8500.
I had spent 10 min trying to get the brightness & black levels to that talked-about level.
Using dynamic mode & 20 cell light, I had finally got it to look decent. BUT it was still not 1/2 as bright as the HU8550, and exhibited a slight flicker that was very annoying.
(This was in a rather dark room)
Also, the vertical viewing angle is rather weak. If you're sitting & watching it @ eye-level, the image quickly degrades as you start to get up.
The black levels/contrast looked surprisingly good on the HU8550.
Mind you, it was in a very bright room set to torch mode, but it seems like it'd do just as well in a dark room with a the backlight turned down.
Of course, as soon as you see a 50" 4K screen vs a 50" HD screen.....you're tempted to go 4K & never look back!