I was in the same boat as yourself about a week ago.
I ended up comparing the 61" and 67" A750 Samsung's to the Mitsubishi 65" 65735 / 6. Everybody has listed good points in previous posts. Also, I want to say upfront that this is my opinion and mine only. You need to take all of this advice and go out there and do a physical comparison.
If this next paragraph is irrelevant to you then skip ahead to the next one. You mentioned a $500 difference between the models that you are considering. So I figured this might be a factor. Of course, you are paying $500 premium for 6" of diagonal real estate. If you compared the Samsung 67" to a Mitsubishi 65" then the dollar amounts almost reverse in Mitsubishi's favor. So if money is a dominant factor then you might consider the 65735 / 6.
I started the process by reading here first. I was convinced that the Samsung LED 67/61A750 was the TV for me. I wasn't even going to consider any Mitsubishi based on the complaints I read about the early 2007 models.
I went to various B&M and checked them out. I first noticed that the picture on the Samsung lamp models looked a little nicer to me than their LED versions. At one store they had a 67A750 setup right beside a 65735. My wife noticed the picture quality difference right away and pointed it out to me. This is from a person who generally thinks a TV picture is a TV picture and doesn't understand the big deal about it. I looked at them myself and I thought the picture on the Mitsubishi had a better picture with more pop and richer colors as well. Also, the viewing angle was definitely better on the Mitsubishi than the Samsung LED. I went back the following day and adjusted some settings and the results were the same to my eyes.
So I had to decide and for me it basically came down to this:
A) I could have a better picture with more pop and richer colors possibly because of a Dark Chip 4. The big drawback is I would have to put up with the lamp maintenance and hope that Mitsubishi has worked out their lamp issues. Price was signifcantly less, although a couple of non-warranty bulb changes could negate this over time.
B) I could have a lesser quality but acceptable picture with a LED light source and a little greener (electricity). I would have to hope that the LED maintenance (lack of) is true and not hype. Price was more too than the 65736.
I ended up ordering a 65736 because of the picture and price. Ultimately, I couldn't make the decision on LED alone. Although it was a major positive factor for the Samsung. If I wouldn't have priced, seen and played with the 65735 right beside the 67A750 I probably would have gotten the 67A750 and been happy. Unless, it arrived and had geometry issues mentioned in the Samsung threads.