uverse is fiber optics and goes up to 24mbps+ (should you decide to shell out the cash)
on topic,
I think AT&T slogan is a little weird. "the fastest mobile broadband network is getting faster." I think AT&T needs to wait until they fully launch their 4g/4g devices before they do any claims. It gets a little old when customers come in the store and ask me why their iphone 4 isnt getting 4g speeds yet lol....
Right now, the best 4g network id have to say is sprints. It has a lot more coverage (they have been out the longest and developing since) and obvious they get the faster speeds.
No one can say how verizons LTE is right now because there are no devices (other than USB modems) that work on it. Those USB modems are getting 10mbps average (according to reviews). The sad thing is that its on a super new - uncrowded network. and the fact that its a USB modem means that the actual cell phones will get a slower speed because of the antenna difference as IOWA stated in a post above.
I wont comment on TMos 4g because I do not know anyone that uses it.
this whole 4g thing is getting kind of annoying and over rated...while having 10mbps download is great, I think in the end it will cause more trouble:
faster speeds mean people will use more data which means carriers will start putting caps (just like AT&T did) on their plans and start charging for tiered plans which leaves a horrible taste in my mouth