sidneylopsides

I'm just going to do a straight side by side comparison, I'll try and leave opinion out of it

Size:
K800i is slightly smaller, it's narrower and shorter, but has a bulge for the camera cover.

For most of the back the N73 is thicker than the K800i, other than the camera cover. The k800i feels smaller in your hand, but not a lot.

Build:
K800i is better, feels more solid. The N73 creaks a bit, and if you push the edge of the front under the keypad it bends quite easily, I noticed that when closing the card slot once. The N73 batter cover moves a tiny amount and that causes nearly all the creaking.
The K800i keypad as a more solid feel to it, and clicks quite loudly, the N73 feels softer, spongier, but is quiter because of it.
The N73 camera cover feels more solid, but is quite stiff.
The K800i keypad is bigger than the N73's
The N73 joystick is stiff, and the glossy finish has mad it slip for a me a couple of times, I have to pus h against the side with my nail occasionally to make sure it's pressed. the K800i is softer and eaiser to move, but also easier to make a mistake with.

Screen:
The N73 is lovely to use, the screen is huge, clear and bright. The colours are great too. The K800i is noticably sharper as it is quite a lot smaller.
The K800i has stronger colours and is brighter, also easier to see in sunlight, especially the standby clock.
The N73 has better contrast and blacks.

Sound:
The speakers on both phone produce a very similar balance of sound, the N73 is much louder, the speakers on top are much better than the ones at the bottom, they sound quite harsh and tinny. This is only really noticable with the 3D ringtones option though.
Call quality, I'd go fo rthe K800i, it's a bit louder and sounds fuller, again the N73 sounds a bit tinny. there's not much difference though.
I haven't tried either with a wired headset.

Speed:
So far the K800i is faster at everything, the main menu, starting the camera, starting music etc. The N73 is faster than previous S60 phones, but still lags a bit.

Camera:
N73: ~4s startup, ~2s shutdown
K800: ~3s startup, ~1s shutdown

Time to focus, take and save an image:
N73: ~8s
K800i: ~4s

Music player start to song playing:
N73 ~4s
K800i: <2s

Main menu:
N73: <1s
K800i: <1s

N73 navigates faster, animations slow K800i down. Opening menu items is faster on the K800i, N73 always pauses for bit.

Review button:
K800i: <1s
N73: ~4s

Now for one I can't time, I might try and video it though, reviewing the gallery is much fsater on the K800i, it's always got a preview. The N73 will show icons at first, it takes a few seconds to show a preview.

3gp video of the gallery speed

Entertainment:
Music playback is easier and smoother on the K800i, the external controls are much better than the player on the N73, which uses on screen controls you have to select. I've also had some tracks stuttering on the N73 when doing other stuff, like starting the camera. This doesn't happen on the K800i.

Video playback
The N73 screen is really nice to view images and videos on, but the playback isn't that good. I sent a couple of clips encoded for the K800i ot my N73, both quite high bit rate, and the N73 stutters. It also won't play a few others I have, which play fine on the K800i. Not sure why.
The video play overall is much better onhte K800i, the cue/review is left and right on the joystick, not up and down, it has more view options, zoom while playing and frame capture.
I don't get the N73 player, left and right no longer control the volume, but they left cue/review as up and down?
I know the N73 will probably get extra video format options, but if it has trouble with these it may have trouble with the bitrate overall.

Camera:

The main feature of them both.
Overall the N73 has great colours and contrast, but very strong noise reduction eats the details, and alot of images have a red cast, expecially low light ones. Low light performance is abysmal. The K800i has weaker colours and contrast most of the time, but more detail and much better low light performance. Oddly, the PhotoDL auto levels will make the K800i levels very similar to the N73.
The controls aren't the easiest in the world, I've seen the shutter mentioned a couple of times, the problem with this is that it's too far into the body, making it hard to press. This causes some camera shake as you force it down. I also find it's a bit too close to the end of the phone, making it more awkward to hold, I'd prefer it where the review button is.
On screen controls are quite good, but not as easy as the k800i.
You can switch to video by pressing the joystick in, but this takes a good few seconds. Pressing up or down takes you to differen menu options, such as scenes, flash, whitebalance etc.
For example, switchingto Macro mode takes 5 key presses on te n73, it's one on the K800i. The dedicated scene/shoot mode buttons make the K800i much easier, as does using the keypad for other shortcuts.
Lowlight previews on the K800i are better too, the framerate is faster on the N73 but ther is alot of noise making it hard to see what you are shooting in some cases.
The low light focusing varies, they are roughly the same in macro, but over around 50cm the K800i AF lamp really makes a difference, the N73 can't pick much up from it's own light after that.
Overall the K800i is much easier to use, faster and produces better images.

Examples:

K800i


N73


K800i


N73


K800


N73


K800


N73


Those are the main points I can think of, ask if there is something else you want to know.