The 6 series TV's are nice as well. We were debating getting that version of our 46", but it was 350 or so more for a slightly better contrast, a 120MHz refresh rate, an extra HDMI port (6 series has 4, 6 series has 3), and a slightly more decorative look. I looked up a couple opinions on 120, and for the most part it seems to be overrated, only really coming into play in extremely fast moving scenes like sports, and even then it's hardly noticeable. YMMV.
So refresh rate alone shouldn't make the decision. For us, the extra things didn't justify another $175 each. We decided to go with a bigger screen in the 5 series rather than a smaller screen in the 6 series since smaller screens make it harder to appreciate the benefit of 1080p over 720p.