I recently sold my Canon 1Ds2 after several years and switched to medium format with a digital back. The digital back files are great, but ergonomics & functionality compared to the 1Ds2 stink. Became clear very quickly that I need a small dSLR to augment the medium format set-up, so I bought a Nikon D300 and a Canon 40D and putting them through their paces. Here are some differences - in no specific order:
D300 AF system is very good, selecting an AF point is very simple and direct. 40D is nice, but the D300 is nicer and better.
D300's metering seems more willing to push the exposure to its edge; 40D seems to under expose around 1/3 of a stop. On the Canon I tend to use EC adjustment more often. D300 needs less EC.
D300 has a slightly larger viewfinder and 100% coverage, but no optional focus screens. 40D has optional focus screens.
D300's LCD rocks. Canon needs to update their bodies asap.
D300's direction pad works better than Canon's joystick controller.
D300's front dial is too far recessed in the grip, it should stick out a bit more. Canon's placement of the front dial (just above the shutter button) makes more sense to me.
Switching metering modes, AF modes and drives modes on the 300D is a bit hard to do without actually looking at the camera. With the 40D I can do this a bit easier because all the buttons are group above the upper LCD panel.
D300 has great build quality; 40D feels equally well bit - perhaps a bit more solid.
D300's on board flash seems better - exposures seem better. Haven't used it much on either camera.
Had some odd WB from the D300; Canon seems slightly better (maybe), but tends to have slight magenta tendency.
40D does better on noise at lower ISO's. On upper ISO it's a toss up and really depends on the light conditions - various conditions mask noise better than other conditions.
Canon's highlight tone priority works as advertised, but I'm not sure if I like it. Nikon's D-Light is need (really just boosts the lower mid tones), but can save some post processing work. Have both in conjunction would be really neat.
Canon files seem need to more post processing, but also take it better. Nikon files need less work in the raw editor (using C1v4 for the Nikon, DPP for the Canon).
The 40D seems to have a tiny bit more DR - may 1/3 stop. It's a tiny, tiny difference.
The 40D shoots at a base ISO of 100; Nikon needs its extended ISO turned on. 40D does better at ISO 100 --- cleaner and more DR.
I'm going to stay away from sharpness because i don't have like quality lenses for both cameras. For the Nikon I'm using the 18-200 VR whereas on the Canon I'm using Contax lenses via an adapter. It's not even fair to compare.
I haven't totally figured out live view yet, but Canon's seems a bit better despite no AF. I have no real meaningful opinion here.
Nikon has viewfinder has gridlines on demand - love that feature.
Nikon shoots 2.5 FPS @ 14 bits vs the 6.5 on the 40D. That's stated in the specs, so Nikon isn't hiding anything, but people often talk about 6 FPS as it's uniformly available.
40D fits my hand better - it sit lower, more in the palm. The D300 are a lip on the rear rubber where the upper edge of your thumb should sit. The Nikon sits higher, slightly above the palm. With the 40D I feel like I can really grip it and I'm not scared of dropping it. D300 would probably feel more secure with the battery grip added.
I really like what Nikon has done with D300, but as said above - it's ~$1749 vs a 40D ~$1119 (I bought mine used for almost $200 less). Bang for the buck goes to the 40D. 300D adds alot of features and with its AF engine, I think a sports shooter should spend some time with one. For more general shooting I think the 40D works better. It's a cleaner simpler design and doesn't get in the way of itself.
Right now I can't choose between them, it's a very close race. I really like the 18-200 VR for its range and stabilization. If Canon had a 18-200 EF-S IS, I'd probably pick the Canon. The money saved on the body could go towards a nice lens like a 35L (a nice fast prime for walk-around shooting). Downside of the Canon is that to match the 18-200 VR, you need a 17-85 IS and 70-300 IS. Canon has cheaper alternates such as the 18-55 EFS-IS and the new 55-250 EF-S IS, so dollar for dollar that matches the VR. VR still wins in terms of having it all in one lens.
Eventually I'll probably buy a 1Ds3 or D3X and flip back to a high end dSLR. Since the D3X doesn't exist yet, I don't which system I'll ultimately pick. I'd prefer to pick the same brand today (ie the 40D or D300) today and purchases lenses which will work on the FF dSLR. In the end there is no clear winner. It's a tough choice.