Welcome aboard YardApe!
By Dish, you mean E* - we do that to avoid confusion with the generic term "dish".

And by ZOOM I assume you mean VOOM - or V*.
And DirecTV is D* - DTV mean Digital TV.
Regarding "signal clarity", that could be 2 questions: signal strength/reliability and Picture Quality.
As for the first, D* and E* are even - for the main birds. E*'s 105 & 121 are weaker - 121 can usually be brought in reasonably well, but until the new 105 bird is launched, it's not good. Do you care? Depends on where you live - where your locals come from. Check out this site:
http://www.dishchannelchart.com/ for that info.
Opinions on D* vs. E* PQ vary - they use different methods of compression, so the degradation that both of them have appears differently - I'm happy with E*, others aren't.
Both D* and E* customer service pretty much sucks - but you can generally get any question reliably answered here in pretty short order. We're good.

Think about it - there's 10,000 members here. Now of course, most are not posting answers, but where else can you go and have say 50 people look at your question.
As for installation - ya gotta watch them like a hawk to make sure they do a decent job. Most especially, lots of hack installers like to skip grounding the gear - that's VERY bad. If you need a SuperDish (to get 105 or 121), those are hard to mount and aim - most guys out there can NOT do a decent job of it (or just won't take the time). Do some reading in the forums here regarding grounding. We have arguments about the details, but ALL of us agree it MUST be done.