This video should give you a good idea on how the screen of the Touch Pro/Diamond compares to that of the Touch HD. The higher resolution on the Touch HD makes pretty much every application more usable because you’re able to see more in one screen, and everything is larger and more readable. That said, it seems that the contrast and color clarity of the Touch Pro/Diamond may be a bit better than the Touch HD (which has a slight yellow tint).
So if your choice is between the Touch HD and Touch Pro, you have to consider what you’ll be using your device for. If you want a flashy phone that is bound to get attention, and one that does multimedia and web browsing exceedingly well, the Touch HD is a good bet. But if you’re more interested in productivity and one-handed usability, the Touch Pro would be the better choice.