I have the exact same situation lol. Here is my thread:
https://forums.macrumors.com/threads/1177075/
I think switching to Verizon is a good idea to retain the unlimited data plan. I am getting the iPhone 4 through the Best Buy BuyBack Program. I think it is the best route to take in order to get the iPhone 5 when it comes out. Here is the price break down for me (you probably have similar)- i am choosing scenario 1:
Scenario 1:
~~Switch to Verizon NOW with 2 year contract and an iPhone 4 @ BEST BUY:
1) At&t Early Termination Fee = +$60
+ 2) iPhone 4 = +$200
+ 3) Best Buy Buy Back Program = +$40
+ 4) Verizon Activation Fee = +$35
- 5) Sell back iPhone 4 to Best Buy in September = -$350
+ 6) Buy iPhone 5 in September = +$700
- 7) Selling my iPhone 3GS = +$200
------------------------------------------
TOTAL COST: $485
Scenario 2:
~~Switch to Verizon with Palm Pixi without 2 year contract (chance of not retaining the unlimited data since it is unverifiable with Verizon)
1) Palm Pixi = $80 (with express shipping to receive it before July 7th)
+ 2) At&t Early Termination Fee = +$95
+ 3) iPhone 5 in September = +$200 (w/ 2 year contract start in September)
- 4) Selling my iPhone 3GS = +$200
-------------------------------------------
TOTAL COST: $115