@ PB PM: Nice shot of a Robin. We have a lot of them here. We have some very sharp photos of them, but the composition on your shot is very nice. I rarely go above 1000 ISO

I shoot wildlife (including birds) with D90 and D300. Probably my best shots are with the 70-300VR. I also use that lens on the D700 but you do have to closer than I often am. All said, I usually prefer the D300 for photographing birds. I rarely use video on birds but do have some useful AVI clips on D90 of eagles in Alaska. I am considering in maybe buying a D7000 as it looks like it will be forever before a D400 comes out. Many feel the D800 is what they want. I personally look forward to a D400 way more. Primary reason is this wildlife issue as in the topic for this forum.

You can get decent results from the D300, at high ISO, but you have to absolutely nail the exposure. Example, D300, AF-S 300mm F4D IF-ED at ISO3200.

I agree with adamz - the issue is the iso with the D300/s. If you don't ever need to go above ISO 400 I would go with the crop sensor to get a denser pixel pattern. I don't like going above 700 if I know I have to crop - 1000 if I do not have too.

Not the best ones and I couldn't find any between 400-2500 but you can see the detail getting washed over.

D300 - 70-200vr f2.8 Hand held - all cropped to the bird (80% +/-) - small amounts of sharpening.
iso 200

iso 400

iso 2500

having both d3s and d300s I can say that d3s is a better option unless You have plenty of light.
I have to disagree with jerl as there's more than one stop difference between those models. I can very comfortably shot with my d3s up to iso 6400, but wouldn't go with my d300s above 1600. In other words I would say that iso 12800 on d3s is similar to iso 1600 on d300s.
Also d3s is significantly faster to focus than d300s, especially with darker lenses, not to mention the higher fps when shooting in 14bit mode (as PB PM mentioned). However, if I have a lot of lights (to get 1/1000 with max iso 800) I prefer to use my d300s as I get a little bit higher closeup.

I cannot speak for the D3s, but I know that my D700 focuses a little bit faster with a TC14E than my D300 did (when using AF-S 300mm F4D-IF ED). The higher end cameras lock focus faster, which is important with fast moving subjects, so that is something else to consider. The D300 will shoot at 8FPS, if you use AA batteries in the MB-D10, so the speed factor is almost mitigated in that case. Of course the D3s has a larger buffer, although I hardly ever ran out of buffer space with the D300. When shooting in 12bit RAW, you get about 20-25 frames before it slows down. Of course the D3s will maintain full speed (9FPS) when shooting 14bit RAW, while the D300 slows down to 3FPS (IIRC), so if that is important, that is a consideration as well.

In low light the D3s will be the winner hands down. I moved to the D700 because in migration season light can be very poor, so I chose high ISO performance over the added reach of the D300. That said, in good light (between ISO 200-800) the D300 will give a little bit more resolution, and is a little more forgiving when you need to crop.

You posted your reply about "diving Osprey" at the same time as me. In this case I think a few extra FPS could be the difference between getting the shot or not. Even with my D3s, I've missed critical shots at some air shows (bigger birds) that I film at.

I'm sorry, I don't own a D300, but after reading Jerl's very valid points, one thing to also mention of course is frame rate. I don't know if you had in mind taking shots of birds in flight, but the extra speed of the D3s might well be useful? As far as your original question regarding image quality between the two is concerned, I think it would be a clever man who would be able to tell the difference just by looking.

interesting jerl that you mention autofocus speed as that is really key in photographing diving osprey, etc. Any other comments on AF speed of D3S and D700 with teleconverters versus d300 af speed would be appreciated.

personally, i like to shoot naked (without teles) and am aiming for highest image quality so i may have to upgrade to fx and work on getting closer to these puppies.

To clarify, i am not interested in 300 2.8 with 1.4x tele versus 600 f4 as that has been discussed elsewhere. i would like to hear comments regarding D3s with 1.4x tele versus d300 for birding. Thanks.

There are several things to consider, so I'll look at them 1 at a time.

At higher iso, the D3 performs about 1 stop better than the D300, and the D3s is about 1 stop better than the D3, so as far as high iso performance goes, the D3s and teleconverter combo will be ahead of the D300. For instance, with the same shutter speed, and with the D3s a stop smaller aperture and 1 stop greater iso, the D3s will have the same noise as the D300 would if it were at a lower iso. With the D7000 instead of the D300, the iso comparison is more fair.

Since the D3s option has a teleconverter, you have more glass surfaces, which will reduce some of the light. A teleconverter, even the best ones, will still have some optical deterioration, whereas the DX frame magnifies the image almost perfectly. In this way, you would expect the DX camera to be at least as good, if not better than the FX frame with a teleconverter.

Autofocus speed is probably more important than any of those options, and is determined by the actually autofocus sensors in the camera itself, the processing power, as well as other factors. Here, the D3s sensors see less light due to the teleconverter, and the depth of field is a bit bigger since it is 1 stop slower, making it more difficult to focus. But I believe the D3s starts out with a bit more processing power than the D300, so I would not be too quick in putting it at a disadvantage.

Other issues to consider is the weight of the body, especially in a gimble mount, and the ergonomics and durability. For instance, the D3s has 2 CF slots while the D300 has only 1. I guess money is an issue as well.

All in all, I don't know that we can give a definitive answer. I would suggest that the best way of finding out which is best for you is to rent both options and do a comparison test based on that.

Assuming 600mm f4 or 300 f2.8 lenses, and the goal is to produce relatively large prints (24x30), which setup would produce the best image quality? Does low light versus bright light conditions change your answer?