What's the difference between Refresh Rates and Refresh Period in a LCD television?

I mean, is it better to chose a television with 120Hz refresh rates or with a refresh period of 20ms.

Refresh rate is how many times the screen refreshes per second. Until recently most LCD screens were 60 Hz. 120 Hz is suppose to work better for the slower odd refresh rate of sources originally recorded on film that appear as jumpy pan shots on older screens.

Refresh period is how long it takes for one image to transition to another. A shorter refresh time results in less motion blur. 60 Hz is a new image every 0.01667 sec (17 ms), so 20 ms would not even keep up with 60 Hz. 120 Hz would be an image every 0.00833 sec (8 ms).