I just made the switch from Cox to ATT, both TV and internet. I really wish I hadn't. I save $15 a month and get more channels, but it's not worth it.
My internet plan went for 9mbps up/ 3 down to 6mpbs up/1.5 down. I didn't think this was a big issue, but netflix is MUCH slower now. Speakeasy speed tests say I'm usually around 5mbps. Uploading pictures to Flickr is PAINFULLY slow.
As far as the TV service, don't get me started. Cox had a MUCH better picture and their hardware was 100% more reliable. ATT uses Cisco boxes that I have had nothing but trouble with. When the installer was hooking it up, he had to go through 4 cable boxes and 2 gateways to find ones that actually worked. My cable freezes all the time, has static, and bad picture. ATT tech support says its my $2,000 TV and my $250 Monster Cable HDMI cable, and want to charge me for a service call to look at the box. I never had one problem with MY equipment (or theirs, for that matter) for the 2 years I had cox.
If I was you, I would stay with Cox.