OP, I very recently [ ~ 2 weeks ago ] was in the same exact seat as you.. LN52B750 or UN55B6000...Both were priced exactly the same!!
I took a chance with the LED and opted for the B6000 ! I knew the 750 was/is a solid LCD and much raved about.. But, having recently bought the UN46B6000, I knew I liked the LED too.. My only concern was severe flashing/clouding.. I was already expecting some, but wouldn't have been able to live with an overly cloudy screen!
Luckily, I got a piece that has very little cloudiness.. And, hence, I have no regrets not choosing the 750 over the B6000!!
If you're buying from a B&M Store Vs Online, go compare them.. play with the settings.. play a BR movie [if the store is kind enough to allow that, which they should considering the big ticket sale] ...
I played with the 630/640 [best buy's version of the 630]/650750, LH90 [LG LED], B6000/7100 [best buy's version of the 7000 series]/8000, Sony V/W series, Toshiba Regzas, Sharp Aquos.. in more than store and several times before I zero'ed in on what I like.. I also compared these to the 8500 even though I wasn't planning on dishing out that much on a TV!
I have also started believing in the Samsung Panel Lottery.. My UN46B600 had a [BN01] Panel and the cloudiness was unbearable.. Off angle viewing [ even a 20 degree shift from dead center] would bother me and get increasingly torturous as I deviated away from the center!!
I took a big risk ordering the 55B6000 from online and ended up with a SQ01 Panel aka S Panel. The whole viewing experience is day and night from the 46" version!! There is still some flashing [ lower left and right corners and some blotching in the middle which is visible when watching with the room absolutely dim or dark, but not enough to bother me.. it's very trivial compared to the 46" which would make me turn the TV off!! ]
Prior to the 46" LED, I had bought the LN46B630 [ AC03 Panel aka A Panel ] which had great dead center PQ, but the colors washed out significantly when you moved away from the center, and even watching from dead center the blacks weren't deep or inky.. I went to FRYs [ B&M store ] and compared my 630 with the one they had on the floor.. I set theirs up the same way as mine, and the PQ was quite different.. The Panel version at FRYs display model was [SS01, also a S Panel]
These two comparisons have somewhat led me to believe in this Panel theory, so even though I hate to propagate this.. I feel morally compelled to give you a heads up on this..
In the end, I would break down the 750 Vs B6000 comparison in two broad points:
1) LN52750 is a more safer purchase in the sense that you'll hardly read any PQ complaints about it. It's a proven top quality LCD!!
2) UN55B6000 is more of a gamble. The probability is 50/50. You may end up with a great unit or one that'll make you wanna blow up Samsung!! But, if you do get the good piece, you'll LOVE it!!!
Goodluck with whatever route you go..