in the size you're looking at you'd be crying if you bought a 720p tv. The image quality would be trash. Go with a 1080p tv, in terms of the hertz, this article has a lot of good info both in hertz and in other info that would be good to know about 60Hz vs. 120Hz vs. 240Hz vs. 480Hz - LCD Response Rate
If you have the money to spend, I'd buy an LED tv over an LCD tv for a couple reasons. First off the brightness and picture quality is inherently better with LED, secondly and thirdly some minor bits are the weight is less in an LED as well as the energy costs are less. Two minor things, but I think they are valid options to think about.
Look through that article as it has some recommendations about TV's. Also Sony and Samsung's pannels are made by the same company in the same factory so the difference is minimal. Tv's are such a big deal now that consumer reports generally does a lot of reviewing on them and i find that their tests are pretty solid and give you a very good idea of where TV's line up, so hit up your library and check out some of their tv articles.
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