Having recently having had both, if I had to choose, I would say that the Directv is slightly more prone to rain fade than Dish.
However, having said that, it is not a big problem with either, and would in no way be a deciding factor for me when deciding between the two.
I currently have Dish, and all else being equal, I prefer the Hopper of the multi HR setup that I had. Programming for national channels is comparable, with D* having the edge in sports, E* in movie offerings.
PQ is pretty close on HD (E* has made an improvement with the Hopper, and IMO it is on par with D* offerings. On SD, E* wins hands down.
The deciding factor for my swap was new customer promos. I could get it as a new dish sub, whereas I could not as a current Directv sub. For you, you will be a new sub with either company, so that is a wash as well for the most part.