Why would anyone stay with AT&T with expectations of the next iPhone having LTE.
Having both AT&T iPhones (4) and a Verizon LTE iPad they are not the same. This past weekend we took a trip that had us in every state in the Southeast US (x FL). In some areas the iPad showed LTE coverage but would not connect. And in others it was slow. (It did absolutely rock on the Auburn campus). And places where AT&T sucked, Verizon LTE was great.
You need to pick a carrier based on what is best for you in the areas you are in most. Verizon technically has my market lit if you count one tower in the downtown area, but AT&T is stronger overall.
As for the tiered data plan, with the 4 iPhones we have 2 are on unlimited, 1 on the 2GB plan and one on the 200 MB plan. So we pay 30+30+25+15 or $100 month. Even if the price stays at $100 and I am limited to 8.25GB, that is better than what I have. My wife is grandfathered on the unlimited plan, but may use 500MB. My daughter on the 2GB plan uses 2 GB. If it wasn't for WiFi at the house, she would blow 2GB away. So being able to just combine our current total data into one pool would be beneficial.