I have had both. Currently I have Dish. For some reason, Dish is the only satellite company that offers local channels in BG. I can't honestly see any difference between the pictures for either provider. UK coverage is the same between the two systems.
Other than the local channels, the big advantage that Dish has for me is that the current DVR's have a Sling Box built-in. That means if your receiver is connected to your home network, you can access your tv from a tablet, ipad, laptop or smartphone from anywhere in the US that has wifi access.That include not only live tv, but also your recordings too. I was able to watch a UK game last year while on vacation in Florida by hooking my smartphone up to the TV. It also has Netflix built-in so it's not necessary to have a separate device to access it.