With the requirements you have listed, the 8830 would be an excellent choice. I am on Sprint and have an 8830 as my primary phone. I have had nearly every model Treo and WinMo phone Sprint has offered since the 6700 came out. To be fair, I really like WinMo phones when I am in the mood to play with different 3rd party apps / tweak the system just to my liking etc....but for day-to-day consistency especially on the email front, the BB can not be topped imo. Add Viigo (or one of the other good free rss readers) and you should be very happy with the BB. The 8830 is excellent as just a phone as well.
Given your requirements, I would trade the Treo for the 8830 in a heartbeat...although I believe you have to have had your billing with Sprint switched over to Ensemble before you can add the $30 plan to a Sero account..not 100% sure as I haven't paid too close attention to that but I think I did read that somewhere.