It seems many people here do not have a clear understanding of the differences between the 2 cameras.
Having used both the D90 and D300s, let me set the record straight one more time.
Image quality comparison
D90 and D300s uses the same sensor. Image quality is almost the same. ISO performance is almost the same. D90 seems better but it is because it is a consumer model and tuned to have a slightly stronger NR. D300s is more of a professional model and it is tuned to retain more details. Yes this is true even at the RAW files level.
What D300s has that D90 doesn't
- D300s has ability to meter with Ai Ais lenses. You can use center weighted or stop metering by default. If you enter the info into "non-cpu lens info" you will be able to use Color matrix metering.
- Magnesium alloy body with weather sealing
- 7 FPS (slightly over 6FPS in real life). Can boost up to 8 FPS with grip and using D3's battery.)
- Mirror up shooting mode to reduce even more vibrations. Quiet shooting mode. Fast Continuous and Slow Continuous shooting mode.
- Faster and more accurate AF.
- Bigger magnesium alloy body.
- You can store sets of settings in D300s, calling them up at will
- Many more buttons for faster access
- Built in interval timer
- 100% viewfinder
- Dual memory card slots (CF and SDHC)
- 51 AF points (configurable to 11 or 9)
- A lot more info and options in Image review mode. When reviewing images, automatic face detection for checking focus on faces, RGB histograms, active focus point indication.. etc..
Downsides of D300s
- Significantly heavier
- Cannot use cheap wireless IR remote for D90. D300s requires you use the remote port located on the front of the body. More troublesome and remotes are more expensive.
- Camera is a lot more expensive.
- D300s' battery grip is a lot more expensive.
- D300s loses all the scene modes from the D90. If you choose D300s, you're on your own.
Should you get D90 or D300s (IMHO)
- If you do not shoot in extreme conditions (extreme cold, wet, jungle, snow, rain), go for D90.
- If you want to shoot a lot with old manual focus lenses (Ai, Ais), go for D300s.
- If you want lightweight, go for D90.
- If you shoot a lot of fast action, sports, or flying birds, go for D300s.
- If you are more of a casual shooter, with a few modern current day lenses, go for D90.
Do not make a buying decision on the two models based on image quality. Because image quality are almost identical. D300s's output is more tuned to the professional, whom are expected to do more PP. D90's output is sharper, with more NR, which is tuned more to the consumer, whom are more likely to just use the pictures out-of-cam. But if you do a lot of PP, you will realize that the output from D300s will be easier to work with, giving you more options. But really, the differences are really really very slight.
Just ask yourself, do you really need the additional features D300s offers?
And always remember the weight of the D300s is no joke. Hold it long enough in a model/wedding shoot and you may find your arms aching and your hands shaky.
D300s is bigger and better. But question to ask yourself, do you need all those bells and whistles? Are you even going to use those features?
HTH.