Unless Sprint switches to LTE and rolls it out everywhere 3G already exists within 6 months (not that it's likely), it's simply not worth a thought...in my opinion of course. Mobile tech changes so fast and so often, I feel it's wise to simply choose the best phone you can afford when you sign up and ride it out until it's renewal time.
Since WiMax is still so sparse, I look at my phone as if it's a really nice 3G phone rather than a 4G device.