As far as the product goes, they both provide pretty much the same product.
As far as sales pitches go DirecTV is much higher pressure and I've heard much more misinformation and B.S. out of them. They've also had a higher number of complaints from clients about deceptive business practices and fine print in their contracts.
http://www.tvpredictions.com/bbb052311.htm
http://www.bbb.org/us/post/directv-customer-service-fail-5588
Dish Network also has some equipment offerings that DirecTV does not. DirecTV only has single tuner receivers. Dish has some dual tuner receivers (two TVs off of one receiver or Picture in Picture on one TV). Their new system, the "Hopper" allows whole house DVR viewing on multiple sets.
You would need to compare the package pricing depending on your wants / needs.
I deal with both Dish and Direct at both of the TV stations for which I am responsible. Dish is easier to deal with and usually more on top of things.