I've been a DirecTV subscriber for years, but not for any particular technical reason, mainly based on available channels for the cheapest package.
The 2 services use different satellites, so trees or other obstacles may determine from the beginning which one you go with. The equipment for the most part is comparable, but not compatible. I know DirectTV has a separate satellite for the High-Def networks, so that requires a larger dish with 2 LNB heads to get the HD and normal channels. Costs extra to have HD service also. Local channels cost extra, and subject to availability in your area. I still just use a roof-mounted directional antenna for the locals and save the money.
I suspect you have been looking at the Dish Network and DirecTV web sites. Examine the programming packages closely to see what channels you get for each, and if the other service might get you your favorite channels for a lower-cost package. That's the most basic feature anyways, the hardware is simply the method to get the programming on a screen.
Other aspects of satellite service that surprises people used to cable:
1. The Weather channel won't display the local radar or local forecast during cut-in periods. That is a service of the local cable company. That alone made my brother cancel his satellite service during a trial period. (He also didn't understand the local channel service, which wasn't available at the time).
2. Heavy rain and snow will interrupt the signal. You will lose service during every thunderstorm. A tree swaying in the wind can also cause outtages if it's tall enough to be in the line-of-sight between the dish and satellite. Considering how many cable outtages I dealt with years ago during weather, it has not been an issue to me, but some people have better cable service than I did.
3. No direct tuning with a cable-ready TV or VCR. Since digital cable service began, more people are used to this now.
4. No local commercials. The satellite provider inserts a ton of their own commercials in the "local" spots. The get redundant. It's really weird to channel hop to the exact same commercial on 3 or more different networks within a few minutes of each other.