The difference between dynamic and native contrast is pretty simple. Dynamic is how much contrast the tv can display over several different frames. For instance, if one screen is pretty bright then the next frame is fairly dark the maximum contrast between these two separate frames is the dynamic contrast. Native contrast which I have seen called on screen contrast is the maximum contrast on one frame. To see native contrast you hit pause. To see dynamic contrast just play a clip that has bright scenes and dark scenes. From what I hear contrast is rarely worth considering in your purchase. As for 120hz, no tv can display a 120hz signal. 120hz wasn't designed to display a 120hz signal either. It was designed to prevent the image from looking jerky. This is most noticeable on blu-ray. Blu-ray is a 24hz signal. If you try to display this on a 60hz tv one frame will be displayed for 3 frames and the next will be displayed for 2 frames. This is called 3:2 pull down. As you might imagine, this can cause the picture too look bad on scenes where the camera pans or with action. The image is basically being slowed and then being sped up. I hear this can make a difference when watching sports shows. To decide if this is something you want, go physically look at a tv with and without it. 120hz will display every frame 5 times in what is called 5:5 pull down. Every frame is displayed an equal number of times and it look smoother. 24 frames times 5 equals 120 as oppose to 24 frames times 2.5 (the average of 3:2) equals 60. I have heard people say that scrolling text does look clearer on 120hz displays, but it isn't that much better. Some people like the 60hz. I will go 60hz because it doesn't make that much of a difference, but I will personally go and look at Best Buy tvs before making a decision.
The tvs you are looking at look reasonable. One thing you should consider is that they cost $99 to ship. The sharp looks like a high quality nice TV. If you notice, several people have already bought the Vizio TV refurbished for Tiger. Why would they have so many refurbished TVs? They only come with a 3 month warranty. I'd consider the Vizio either a really good deal or a waste depending on whether you have to pay to get it fixed in 18 months if it were to break down. The sharp has a 1 year parts and labor in this case and is a solid investment. The sites I am looking at for TVs are newegg.com, tigerdirect.com, and crutchfield.com. Tiger rarely has free shipping, but I trust all three.
Here is the same Sharp TV your were looking at with free shipping:
http/www.newegg.com/Product/Product.aspx?Item=N82E16889101173
This Sharp and the Sony I linked to earlier are bound to be two of your highest quality options. If I had to order TV manufacturers in order of quality it would go like this: Sony, Samsung, Sharp, Toshiba, and then all else.