I can confirm no throttling by Sprint. Since getting the GN2, I''m using over 50GB of data per month. It's up from the 25GB or more when I had the Evo 4G and used 3G almost exclusive; I had access to WiMax if I went to certain towns. I live in area LTE, but it's not everywhere, so I bounce between both all the time.
The other big difference, Sprint is CDMA and has the simcard built in. ATT is GMA, which is used world wide. ATT may not have the simcard build in, and thus world travel friendly. Something to consider if you plan on traveling.
Beyond that, I would just look at your data usages, carrier coverage, and reliability.
Lastly, you do realize we're going to be bias to Sprint.