Allow me to add my experience as follows:

a) Although the N900 is slightly dimmer,

b) Color-wise the phones are quire similar. N900 seemed a bit better but it could be bias. Same goes for HD2.

c) My N900 runs the birds like nobody's business. I haven't had a single slowdown unless there was a call or notification. It might be your phone that runs stuff.

d) I saw the app run on both. There is no performance difference in either, the animation is smooth. I have a water cooled quad core with a dual GPU at home. I know framerate when I see it. It was smooth. On both.

e) the iPhone is not exactly big on multitasking. N900 is. If you have performance differences it's probably that something's not right in the background.

Oh, and, the 24 bit (16M color) comes from the specs. What really is displayed can be tested relatively easily. I will run a test tomorrow.