Cost, programming, and equipment are the factors. For cost you just need to look at the packages you want on each and compare. Make sure you consider the prices after the promotions go away, unless you want to switch every year--which isn't the worst strategy. As 118 says, Directv may have an advantage with including the Big 10 network while Dish requires you to get the sports package. The only funny thing I see about Directv is that ESPN classic is only on their most expensive package, but I rarely watch that.
Programming also depends on where you live. Can you get Fox Sports Midwest on Directv if you're not in that region? You do with the Dish sports package. That's one reason I stayed with Dish, but it was a bigger deal when we might have Big 12 games on Fox Midwest that weren't on my regional network. Plus that's how I'd see the Bo Pelini show, and a few basketball games. In the Big 10, I might not even care about Fox Midwest. Also there are things like 118 mentioned about NFL and MLB networks not available on Dish, if that's something you want.
For equipment, I only need one receiver with Dish. I get HD on my main TV, and can distribute a separate SD signal to the rest of my house, with help from pre-wiring I had put in my house to have an in and an out cable where the box is. I can watch one thing on the main TV, and someone else can watch something else on another TV, without another receiver. With Directv it looks like every TV needs it's own receiver, unless use a splitter and want to see the same thing on every TV. You may even be able to split the HD signal by using HDMI to one TV and component cables to another, which I've done with my Dish receiver. So you have to think about how many TVs you have and whether you need HD everywhere, whether you want them individually controlled, and the cost of each receiver you'll need.
So I have stuck with Dish because I could get away with using just one receiver, being able to get all Fox/Comcast regional sports networks, and because I don't care about the NFL or MLB network. I'll have to go through the exercise to see if it's worth it to switch now that things have changed.