Even by only looking at the specs, you can already tell who is the winner here - Nokia N80. At its highest setting it can record at 352 x 288 pixels (CIF) resolution, while Sony Ericsson can only shoot video in the ancient QCIF format - 176 x 144 pixels. So, N80 captures 4 times more pixels and its video is definitely better. Take a look at the samples.
Both phones can zoom while capturing video and both can record audio and the video recording time is limited by the free memory only. Sony Ericsson features digital video stabilization, but it's pretty redundant at such a small resolution. Anyway, both cameras will miserably fail when compared to the newly announced Nokia N93, which will feature recording DVD like quality video (640x480 at 30 fps).
Nokia N80 and Sony Ericsson K800 score about the same here. Both phones have good equipment for viewing the taken pictures - fast processors and high-resolution displays. Of course Nokia N80 has a resolution advantage. Both phones can show the pictures taken in a landscape mode, utilizing this way the displays at full extent. Nokia wins a point here with its picture browser Gallery. It reads ahead the picture information and you can go through the pictures really fast.
When it comes to zooming a selected picture however, Sony Ericsson K800 is a lot faster. It seems that the Cyber-shot technology helps in this task. It is annoying that both phones have a lot of zoom steps. When you want to check the focus of a given picture you don't really need, say 1.25x zoom, you just want to see the details as quickly as possible.
So, which camera is better? In our opinion Sony Ericsson K800 takes overall better pictures even with the beta firmware. The pictures taken with K800 look more natural and the camera gives reliable results under various lighting conditions. Not exactly a big surprise taking into account the years of experience and know-how Sony has with its Cyber-shot line of digital cameras.
Nokia N80 shows that it can be an adequate competitor and far exceeded our initial expectations even without the Carl Zeiss logo and the autofocus lens. Overall it is a better imaging device than Nokia N90 and Sony Ericsson K750. Nokia N80 also wins in the video capturing department. It's up to you to decide whether this is important for you and depends on what you are going to use your camera phone mainly for. Not that it takes perfect video records, but K800 doesn't offer more than the current absolute minimum.
And about the upgrade from Sony Ericsson K750, is it worth it? If you are going to print your images - definitely. The additional resolution will be visible even in prints 10 x 15 cm (4" x 6"). Even if you only publish your images on the web or look at them on your PC, the down-sampled images from the 3 megapixel cameras generally look better than the ones from K750.