I maintain subscriptions to both TWC and D*. As said above, cable is highly dependent on your area. In my area DirecTV is a notch or two better then Time Warner on most HD channels as far as picture quality is concerned. On the premium movie channels they are dead even, I tried and I cannot tell the difference between the two. Also, while DirecTV has the most NHL CI/NBA LP/MLB EI/MLS DK games in HD, I have to say Time Warner's picture quality is a tad better on this. While DirecTV is MPEG 4, most cable systems use MPEG 2, but in a lot of areas TWC uses MPEG 4 for the less viewed channels like international and season sports subscriptions. Where there are a lot of NHL and NBA going on at the same time is when I usually notice this. Downside is the most TW will have is 9 games in HD at the same time of a sport. TWC seems to have more pronounced problems with 'black crush'. On DirecTV when I notice black crush, I flip over to TWC and it's usually a little worse. Although I've noticed that seems to come and go over time. I used to have awful problems with that on FX on DirecTV years ago, before I had cable again, but it subsided.
I do like Time Warner's hardware much better than DirecTVs. I've had the HR34 since it first came out it was nice back then, but it has gotten so slow over time, sometimes it's almost unusable. My Arris 3600 is lightning fast and a noticeable improvement from my previous Cisco boxes, which I thought were decent. Also, DirecTV's HD UI doesn't take advantage of the additional screen real estate. Six channels with 90 minutes worth of programming. Time Warner has seven channels with two hours worth of programming, and best of all NO ADS! Six tuners on TWC versus five on DirecTV is also a plus. It may not seem like a big deal, but on many nights I have 3-5 things recording simultaneously, while watching a live hockey or baseball game. Or if I have extra tuners available, keep an eye on two games at once with multiple live buffers.
Both services have their pluses and minuses.