I'm guessing you are talking about Ipad, or do you have new Iphone already? It would be hard to make any direct comparison with Ipad, but as dark10x mentioned, there's already been some clear signs that ipad has less fillrate at its disposal than 3GS for the gaming types of stuff - and new Iphone could suffer even worse thanks to a chip that likely won't even run at 1GHz like the Ipad one does.
As for the screen being darker, I know that it's not on Ipad, but that's also physically much bigger screen, with different and bigger backlight etc. I was looking at some screens on Engadget from the Iphone 4 announcement and that's where I saw this. In all these screens 3GS is on the left. But then again, it could be just the camera angle playing tricks for all I know.
[ig]http://www.blogcdn.com/www.engadget.com/media/2010/06/apple-wwdc-2010-185-rm-eng.jpg[/img]
[ig]http://www.blogcdn.com/www.engadget.com/media/2010/06/apple-wwdc-2010-180-rm-eng.jpg[/img]
[ig]http://www.blogcdn.com/www.engadget.com/media/2010/06/apple-wwdc-2010-182-rm-eng.jpg[/img]
*edit* Then again, on this other photo made later on, it seems the exact opposite. As you said it's hard to say without knowing what their brightness slider is set to: