To be honest as long as those two companies do not throttle your connection or impose a download limit (doubtful in both cases as you will be on a business class connection) I don't really see a downside to either. I would place my hand in Time Warner simply because I have a dis-like of AT&T, but I've come to understand TW can be just as bad. It's a bit like picking the lesser of two evils I suppose.
No throttling is the reason for going with business class service. That and the hard sell for bundling seems to be a bit less. I have no love for either Time Warner or AT&T and I figured I could find pretty balanced dislike for each, hence my request that this not devolve into a hate fest.
How's your current service with AT&T? Many times you can tell what kind of service you will have based on the regular consumer in the area, especially in cases of non dedicated connections. So if AT&T is working out well right now, I would probably upgrade with them. I would also suggest seeing what your neighbors say about TW.
My AT&T service has been pretty good until recently. I've had three or four outages in the last 8 years. Their support is pretty useless until you make it through to the second level tech guys. Nothing unusual there. I've never been without service for more than a day. Of late I have started to see some inconsistency in my connection throughput. Most of the time I get advertised rates (5Mb) but from time to time it will drop off greatly. Not really an issue when surfing, but Netflix and Amazon Instant Video really don't like it. Because it is so intermittent, I don't have a whole lot of hope regarding getting AT&T support to help me troubleshoot.
A lot of it depends on your location... For example, I've used TWC here in NYC, but switched to Verizon's FiOS many years ago because it gives me better connection stability, better speed and better customer support compared to TWC. In your location some of these things may be opposite... I suggest going to DSLreports' forums and reading up about both of these providers, how they operate in your service area.
Also, who is your cellphone provider? If it's AT&T - you might get additional discounts if you sign up for U-Verse with them...
I'd love to get FiOS, but I'm not "served" by Verizon. I've wandered through the DSLReports forums to some degree and unfortunately there doesn't seem to be a lot of current input from the DFW area for some reason. Who knows, maybe I haven't looked hard enough. While input from other areas does help with the overall trend, as you note, service and support vary greatly by region.
Thanks to all for their input.