Just the other day, Sprint announced that austin tx, along with 100 other markets will have lte released before the end of 2012. With lte, you should get blazing fast speeds (anywhere from 5-30mbps down) compared to what you were experiencing on the iPhone 4s. Once Austin is done, the 3g network will be upgraded too and with sprints advanced 3g you should be getting speeds between 1-2 mbps. If you want my opinion, save money and switch to sprint. The network may not be perfect right when its launched, but it will progressively get better and for unlimited data, and the price, you can't beat it.
-Ryan