Any way you put it, the D90 still has better high ISO performance than the 300s. People may try and write it off as a menial quality, but it's obviously a pretty big deal.
You told the guy you quoted to back up his statement with detailed information so I'd love to see the detailed information that shows the D90 high ISO performance advantage over the D300/300s is anything more than a negligible difference.
The most obvious, and easily accessible information can be found at DXO labs.
They are factual, and not opinion based so it's a good place to pull information. Of course when considering their results, you have to take into account you are looking at raw based information (which most of us shoot).
If you start off looking at their rankings of all cameras based on sensor performance, the D90 comes in at #17 on the list, one point after the Canon EOS 1D Mk IV, while the D300s comes in 21st, and the D300 24th.
They rate the D90's sensor overall at 72.6 out of 100, the D300s 69.8, & D300 at 66.6 respectively.
That number takes into consideration overall sensor performance (ISO, color depth, dynamic range), and not just the ISO.
The D300's comes much closer to almost matching the D90's sensor capabilities (and the D5000's while we're on subject), but still loses out on every measure of performance for the different categories.
DXO states that with a 5 point difference in sensor score, that corresponds to a sensitivity gain of 1/3 of a stop. So with the D300, the difference in not only High ISO performance, but sensor performance overall is noticeable.
For High ISO performance they rate the D90 at 977, D300s at 787, & D300 at 679. These numbers are by far the greatest variation when looking at the test results.
They have the Nikon D3s rated at 3253 for comparison.
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I also stated, as you quoted, that there was a difference... I'll add, no matter how much D300(s) owners dislike it.:er: