ive owned the k800i before so i could give you my opinion on it....
First off, its a GREAT phone, there weren't really many flaws to it at all in my opinion, i'm with T-Mobile so the EDGE and 850 was useless to me anyways, i dont travel around the US that much so i wasnt worried about it.
It has a great screen, a huge improvement over the k750i...
I assume that your main concern in these phones is the camera, because they both take good pictures, the k800i has the xenon flash > the n73, while the n73 has the carl zeiss lens > the k800i. Well if you go to this link, you can see picture comparisons between the 2 phones, http://www.gsmarena.com/n73_d900_k800_photo-review-102.php , in my opinion, the N73 takes better daytime pictures is terms of clarity, but at night time, the k800i wins because of its flash....
I used to own a k800i, and i sold it so i could buy a Sharp 903sh... and if you're really looking for good camera quality, then the 903sh is where you belong, the picture quality of it is incredible, it will spank the k800i in daytime in color and terms of clarity, but once again, the k800i will do quite better at night time.
The video camera on the k800i, i feel, is not even worth mentioning, my old Samsung e720 could take far better video with its 1mp camera. So its pretty much useless especially since there is no flash while taking videos either.
I think that the k800i looks better than the N73, but thats just my opinion.
I've seen the N73 in person, its size is very close to the same size as the k800i.
I've also been reading online that the camera cover of the N73 is very loose and sometimes its better just to keep it open while its in your pocket so it doesnt open in there and waste battery life without you knowing.
Hope this little mini review helped