Your in luck!
I went to sprint store today to take care of a billing issue and picked up a brochure of pro and diamond. (1 brochure)
Basically you get full hard keyboard on pro where diamond just gives you the touch qwerty keyboard.
(If you have big monkey hands, sausage fingers or claws go with the pro.

)
Pro also gives you a slot for microsd slot for expanded memory but no internal memory and 512mb of rom instead of 256mb. Diamond has a burgundy back while pro has a silver back.
That's about it. They use same touchflow 3d, winmo6.1 same
Oh wait!!! IT's much FATTER then Diamond.
I like my diamond slim.... sexy... LOL.
Hope it helped buddy.