I'm not sure where you all are getting your facts from about Sprint having the fastest data network available. The current Sprint EV-DO Rev. 0 network has a maximum download speed of 2.45Mbps, and a maximum upload speed of 0.15Mbps. While Sprint has the Rev. A network (3.1Mbps Max Down, 1.8Mbps Max Up) available, support has not yet been rolled out for it.
The HSDPA network has a maximum download speed of 14.4Mbps. AT&T is currently only utilizing 3.6Mbps max download speeds, but that puts them ahead of even Rev. A which isn't available right now. AT&T has not rolled out HSUPA support yet, which offers max upload speeds of 5.76Mbps, but the current average for uploads speeds is 0.1-0.2Mbps. So AT&T's current average upload speeds are faster than Sprint's current maximum upload speeds.
And yes, the AT&T Tilt currently supports 3.6Mbps download speeds through an HSDPA connection. Obtaining HSDPA is as easy as a one-line registry edit. GPS is fully supported right out of the box.
I will admit that AT&T's service is a bit outrageously priced for what you are getting, but they do currently have the faster network. I don't know where any of you live, but in my area and anywhere else I go, AT&T's 3G support is great.
All my information was found in the links below.
http://en.wikipedia.org/wiki/WiMax#Comparison
http://en.wikipedia.org/wiki/List_of_HSDPA_networks#U