I dont know that I can prove this, but Im pretty sure that AT&T was throttling my "unlimited" data plan well below the 2GB limit. It was bad enough that I rarely used more than 1GB of data per month.
Fast forward to my iPhone 5 upgrade. Granted there is decent LTE coverage here in Houston, but with no change in my usage habits I hit the 90% mark of the 4GB plan(I needed tethering, so I had to "upgrade") 18 days into the billing cycle. My main data consumption was iTunes match over cellular. I was getting constant buffer underruns with my iPhone 4 on 3g, but now Im hitting the data wall halfway through the billing cycle. Its now in AT&Ts best interest to open the pipe for me so I can pay overage fees.
I dont know that it would be better on a different carrier, but I certainly harbor no goodwill towards AT&T. I feel like Im one hassle away from buying out my contract.
Just getting my iPhone 5 proved to be a multi hour headache due to an AT&T agents royal screwup of my initial upgrade. I ordered a 64GB black phone, was shipped white. He also updated my home/billing address incorrectly(more on that later) I returned it, and they spent 2 hours reversing the upgrade and giving me a new sim for my iPhone 4. That took so long that I couldnt re-order the upgrade. A week later I used the AT&T website to order the new phone, however, it could only be shipped to the billing address that the AT&T agent mistyped when I originally ordered the phone. So I had to cancel the order. But it wasnt that simple. I am a "premier" account and premier support isnt open on weekends. I had to go to an AT&T store and cancel the order. Then I had to call premier support on monday and reverse the upgrade. The next weekend, they finally had some black 64GB iPhones in stock. I bought one. Guess what? it had an intermittent power button. I had to set up an appt with an apple store to get that fixed. Fortunately the Apple store experience was much better. So 3 iPhones later, I have an iPhone 5 with a bad carrier and have to now worry about my data usage. First world problems I suppose.
Also in Houston here, also had AT&T 3G service. I doubt they were throttling your old phone - their local 3G network is just that bad. Using AT&T's LTE network in the same town is rather like using a different carrier. Even the HSPA they built on top of their old 3G is remarkably better. And it's considerably faster than Verizon's current LTE offerings, which have been degrading in speed/quality since mid-April. (Though that might improve in time, and MaBell's service quality might go back down as more people upgrade to it.)