| | Re: help me decide
All good advice. But really consider the Q10. It is much like the 9900 and your 9780 in that it has a keyboard and a square screen. The screen is larger because it doesn't have the trackpad and buttons. That may be a factor in your decision as well.
The Q10 has all the latest technology available from BlackBerry and will give you greater capability going forward. The keyboard is very nice, even for a BlackBerry, and the battery life should be very good, based on my experience with the Dev Alpha C (development prototype of the Q10). The 9900 on the other hand will give you the BIS based BlackBerry communication experience you are used to, but with more power and a touch screen. The legacy devices are likely to see less support from application developers as time progresses, though you can count on the same level of support from BlackBerry for quite a while.
My other Blackberry is a PlayBook.