Dumb Video DSLR question

I have a dumb video DSLR question. Do the Megapixels of the sensor for video in the new video DSLR's make the ultimate difference in video quality of the new video DSLR's or is it related to the resolution of the LCD screen? My understanding of video in these cameras is that it is essentially the LCD screen feed that you are seeing when recording video. If the LCD feed is the limiting factor, is there any value to a sensor with resolution beyond that of the LCD screen? Would one be better off with a lower MP camera (as long as it not less than the LCD resolution) for faster processing speeds etc?

If I had a 1080p recording from a 12MP camera would it be the same as that from a 24MP camera?

The quality isn't really dictated by either the sensor resolution, or the LCD resolution. In fact higher resolution sensors actually make the video quality a bit worse in some circumstances. If it's full HD 1080p for instance, it's just subsampling pixels from the sensor, so for the 5d mark II, I think that would be one in every three lines of vertical resolution. Ideally, if it were a dedicated video camera, it would be better if the sensor was exactly 1920 by 1080 pixels. So, in theory the video from a 12MP camera would be better than that from a 24MP camera and video from around a 2MP camera would be the best, all other things being equal.

There has been some discussion in video DSLR forums about the aliasing, moire etc caused by the subsampling from high resolution sensors, but I havn't really seen it yet in underwater videos. It seems to be worst with straight diagonal lines, and stripey shirts etc. Not something you normally encounter too much of underwater.

Depends on how the sampling is done. With the 5D2, it skips every 3 lines due to a smallish processor that has to squash 21mp worth of info into a 2mp space. That's why you have aliasing and artifacting. If they could squeeze a i70 Intel chip, the story would be different. It's also about the codec. A 21mp sensor sampling badly will give crappy results, while a 6mp sensor with the right encoding chips would do better. Theoretically, you should get 4-5k resolution out of the 5D2 sensor IF it had a super powerful processor.