I have the TN. It's great. Not as good as IPS, but nearly as good for straight on viewing which is how I use my iMac. ConsumerReports rated both screens excellent last time around.
I've never noticed the contrast shift in a movie. The only time I notice is if I put a certain color at the very top of the screen and the same color at the very bottom and look carefully at each color a few times. For example, an orange will have a slightly more yellow tint on the bottom of the screen than at the top. It is not something you easily notice unless you really look for it.
But it is pretty much impossible for your eye to take this all in at the same time. And in a movie it is even more impossible to see the difference because the scenery is always changing. The only way you'd notice is to pause the show and look carefully.
That being said I'd probably get a 21.5" iMac because there are many more improvements and extras compared to the old 20" besides the screen.
For the extra $200 or $300 you get a slightly better screen, slightly bigger screen with greater resolution, bigger hard drive, faster cpu, 4 memory slots, aluminum back that you will never look at, better speakers (quite noticeably better bass,) magic mouse wireless, wireless keyboard, brighter more energy efficient more responsive LED lighting and according to Apple quieter and better heat dissipation. Did I miss anything?
20" is nice though. Using it right now.
