Both will try to tell you that their service is better for various reasons. Both are lying. They only promise "up to" their advertised speeds. You can ask them what the minimum guaranteed speed is-they probably won't promise anything.
You may be able to search online to see what other people in your area experience. For all the technology they advertise, the only thing that matters is the speed they actually deliver. Uverse and Cable are both capable of giving very fast service or very poor service.
One other thing to consider is upload speed. If you use dropbox or upload videos to youtube or anything like that, your speed will be limited by the (usually unadvertised) upload speed. My guess is that ATT offers 3Mbps upload speed at the tier you are looking at. Time Warner probably offers a higher upload-maybe 10 or 20. Neither will be guaranteed, however.
Time Warner's 200Mbps speed is almost certainly faster than 45Mbps on Uverse. In my experience, Time Warner is the devil, but they will give you faster service than ATT for less money. Both will raise your rates through the roof after your first year. While Time Warner is the devil, they are probably the lesser of two evils.