For the kind of work I do, the Blackberry was overall a better fit. The keyboard is bigger, the cradle allowed me to feel paging better, a slightly bigger screen, and I can do over the air syncs for my calendar, task list, contacts, and notes.
However, yes, the tradeoff was:
1) I lost a great quality screen (the BB 7750 isn't as bright or sharp as the T650 but it gets the job done)
2) I lost SD card space (you can expand the T650's storage by buying little flash memory cards with 1GB of space and filling them with files and even programs -- I've seen T650s used as MP3 players, while the BB 7750 is limited to 14MB of space)
3) No camera
Both do internet well
Both do IM, though right now neither of them "includes" a program that does the IM -- you have to purchase it, if I recall correcly. One time purchase, no big deal
Both do email and paging, though the Blackberry does it better, in my opinion.
Both can do calendar and task lists
T650 has more games and entertainment options available for cheap/free