My second-shooter wedding work for friends that shot Nikon systems gave me opportunities to take their gear (initially D200s, now D300s, with Nikkor pro zooms) for a spin. During a wedding in June 2008, I mostly shot my E-3 with the 14–35 f/2 and FL-50R but took a bunch of formal and reception shots using my friend's D300 with the 24–70 f/2.8 and SB-800. I also took a bunch of her raw files home to compare to those from my E-3.
I was certainly biased from months of use, but I preferred the control layout on the E-3. Everything is a button and a dial and easily programmed via My Mode and Custom Reset functions. The D300 has a combination of switches, levers, buttons and dials. Then there's the interlock and ring to control the drive (single, continuous, etc.). The D300 puts the ISO button to the left of the prism, so you need to move your left hand off the lens to change the ISO. I change the ISO frequently, and doing so doesn't require moving your left hand on the E-3—it’s done with your index finger and thumb. Nikon also doesn’t include auto-ISO as one of the ISO settings—you need to access it through the menu system. It’s the one under ISO 100 on the E-3.
The 2:3 viewfinder seems weird. It's great for group shots but odd for many portraits. One argument you could make is that 4x6 photos comprise the bulk of photo sales, and the ratio does match this size perfectly. The long, narrow frame seems to give less options compositionally and for cropping during post-processing.
I like that Nikon puts the arrow keys under your right thumb, which makes it very easy to move the focus point. Olympus puts the playback button in that position, which is a big gaffe. It's not a shooting control, so it shouldn't get such a prominent position. Nikon puts it to the left of the prism where it belongs.
The main image quality differences that I saw between the cameras is that the D300's blacks are a lot blacker and the overall image looks crisper. The D300’s noise floor looks lower by about two stops, particularly in the shadows. A black tuxedo looks black in D300 images while it shimmers a bit from the noise with the E-3. There's also more room to lift the shadows and recover detail with the D300 since the noise doesn't become objectionable as quickly. Lifting the shadows in an image from the E-3 dredges up noise and banding regardless of the ISO. Images from the E-3 always appeared a bit soft to me and didn't respond well to sharpening in post-processing. Not so with the D300. They're crisp from the get-go and sharpen up nicely if needed.
Both viewfinders look good, but the E-3's is brighter with that beautiful f/2 lens mounted. Of course, Nikon sells a range of primes that are even faster and brighter. I do like that the D300 displays focus points using an LCD overlay on the focusing screen instead of etching them into the glass. This way, you only see the active AF points and can turn off the rest so they don’t clutter up the viewfinder.
The D300 has a beautiful rear LCD that's better than the E-3's for reviewing images. But it doesn't pivot like the E-3's, which seriously compromises its utility for live preview. Olympus is ahead of the game in this department.
AF performance on static subjects in good light is similar on occasions when the E-3 is cooperative (see E-3 focus troubles), but the D300 is way faster and more accurate in low light and on moving subjects. I never had to wait more than a second for the D300 to focus at the reception, while waiting through 4–5 seconds of hunting was common for my E-3, and even then correct focus was a hit-or-miss proposition. When not using an external flash, the D300 also has a proper focus-assist light built into the body. The E-3 flickers the internal flash if you’re not using an external flash, which annoys the hell out of everyone. The D300 also has no trouble tracking people walking down the aisle in low light, which is something that the E-3 simply can’t do. Actually, let’s be honest here: the D300’s focus gives the E-3s’s a thorough spanking and sends it to bed early.
It's pretty crazy comparing the Nikon 70–200 f/2.8 and Olympus 50–200 f/2.8–3.5 side-by-side. This is where the smaller sensor really pays off. The Olympus lens is significantly smaller and lighter and offers 1.5x the reach, since the D300 uses a cropped sensor, with only a slight penalty in aperture. Too bad it’s the only place where the smaller sensor pays off.
The D300's DX sensor puts you in an odd place with Nikon's lens lineup. Nikon doesn't offer the exceptional line of lenses matched to that sensor size that Olympus does for 4/3. In the pro glass category, you're largely stuck buying full-135-frame lenses and dealing with the crop factor. This is great on the long end, say if you’re a bird photographer, but makes a mess out of wide-angle and mid-zoom lenses. A 28–75 becomes the equivalent of a far less useful 42–112. I think it’s worth seriously considering the D700, which I found to be a real winner for PJ type applications.