You have a T5 and considering a 650; ah I am in the same boat, though my pracitce device has been a 600 for a while. I would say go for it as you will grow quite used to having lighter pockets, and even lighter if you get a BT headset.
For me personally, if I can get a GSM 650 for under $300 used withuot getting a new contract, then I will get one. Else I will have to wait and see if another Treo will come out that will better suit my needs (Drive mode and 256MB of RAM are needs after having a T5 ya know

)