HTC One X on ATT's network looks better! Plus its they have better coverage! (Pending on location.)
Lets go down the list of 2 devices...
Kickstand - I Hate the kickstand. Its annoying nor it is something that I would use. I know many people love this option and I am happy they enjoy it. But truth be told...A case can fix this issue. When I had my Evo 3D I was glad they gotten rid of the kickstand. But the ones who wanted it got the HTC case. So this is not a huge difference.
The look of the 2 devices. - Sorry but the ATT One X just looks better. White or gray. Sprint drop the ball here! But again...A case can fix this so its not a huge deal in the end.
MicroSD Card - This is what really sets them apart. Sprint did a good job here. The One X needs this option. So Sprint wins here without question.
As far as the service goes? This will only matter based on your location and your needs.
Data - Sprint offers unlimited and ATT will give you 3Gigs for the same price ($30/month). For me I rarely go over 2gigs a month. Even while streaming Netflix on my 3D. But the fact still stands...ATT offers a lot better 3G connection. In my current city I am getting 7Megs down. With Sprint I was only getting about 200K!
So in the end ATT won m over. I was a Sprint customer for a long long time. But I am also happy to report that I do not regret moving on! So thats my .02! Hope it helps. If you go with Sprint I hope they serve you will!
Another option that you have is to buy out your ATT contract and head on over to Prepaid and save even more!
It boils down to what you personally value. I was with Sprint for 10+ yrs and never has any issues with drop calls and 3g until they got the iphone. Their 3g was so fast that I never needed to turn on 4g. Once they got the iphone, I had to always turn on 4g to do basic surfing.
Sprints monthly and excellent cust serv was great too. Kickstand, micro sd card and dedicated camera button arent that big of a deal too me. Never used my kickstand on the evo4g and the camera button to launch the camera was too slow. I also dont load my pda up like a mp3 or video player either.
OneX launches the camera and takes pics in under a sec. Its so fast that I take two pics bc it seems like I didnt take the first pic, its just that fast. This makes the camera button a mute point.
If I really wanted a kickstand, HTC makes this really nice OEM kickstand case for the OneX. Mutes the kickstand issue.
Got my monthly the same as sprint with a family plan and 24% corp discount. Mutes the monthly. Never went over a gig of data on sprint, so that mutes the data issue.
Sprint doesnt have LTE yet and at&t does. Their LtE is so fast it feels like your on a wifi network no matter where you go. Im blown away with how fast lte is. Plus I prefer the OneX design wayyy more.
Never thought I would switch, but I did and do not regret it. I do still stalk the sprint forums bc I had sprint for so long and never had issues until their 3g tanked and now no 4g. The EvoLte is such a nice phone to be limited to sprints current 3g. I know It would of frustrated me so much. Im glad I switched but will miss Sprint.