Ok, as someone who had DirecTV and switched to Comcast two years ago I thought it was time to put my two cents in.
HD Picture-DirecTV wins, but Comcast here comes very close, I live in a rebuilt Comcast area(Metro Detroit), very few analog channels, almost all Digital, so plenty of room for channels, no blocking or pixels, just a very, very good HD picture, but D* is a tad better.
HD Channels-Comcast wins, here in my area they have more then D*, like how D* just got E Television, C* has already had it(never watch it ), they also have more Premiums in HD, all the HBOs in HD, the Encores, Starz, etc, the only one they don't have is HD Net Movies, but they do have BBC America HD and my wife will kill me if she loses that.
Price-Comcast, Triple play is well worth it, I now pay about $157.00 a month, that includes 2 HD DVRs(first one is included in the package, second is $4.95), 2 HD Boxes(also $4.95 each), all the channels and 2 premiums(HBO and Starz), Broadband, Home Phone.
The only way D* would come close to that price is if I got the entertainment package with 1 HD DVR, 1 HD Box and just HBO, which came out to $60.00 a month the first year, then Broadband and Phone would be $99.00 a month from Comcast.
The Hardware-DirecTV wins, the menus, guides, etc on the boxes are leaps ahead of Comcasts boxes, but the boxes from Comcast are fine for basic info and for DVR programing.
As far as Cable going out, I have had only 2 outages in the last 2 years, the longest being 2 hours, I know sooner or later there will be a massive storm and it could be out for days but it has not happened yet, on the other hand every time we had heavy rain, D* went out.
On Demand-Comcast wins, there is so much on, Comcast really has this On Demand thing down just right, so much programing, from tons of free movies in HD, TV shows, etc., and if I forget to record something, it is always on On Demand the next day.
Now this is from someone who had D* and loved it, I had no problems with it, no complaints at all (except no BBC in HD), it is just that Comcast works out for me better for pricing.