back years ago, I saw a map that indicated that Dish had centered their aiming from the satellites on NC - (you could get reception with a 12 inch dish in ideal weather conditions !) But that was probably only for 119 and 110.
Losing reception - I can't say about DirectTV - I've never had it. OTOH - I have lots of experiance on Dish, both main arcs (Eastern (61.5, 72.7, and 77) and Western (110,119,and 129)). With a properly aimed dish and cables in good condition - Loss of Signal doesn't happen very often - and when it does - you usually have more important things to think about (like - where am I going for tornado / severe thunderstorm protection). This is exactly why I always try to have alternate means of getting local news stations (OTA antenna since I mastered that).
Frequencies for the two services - the basic download range was the same for both DBS providers - back in the early days, you could actually have both systems working together. (Directtv had service on 101 and 119, with a couple of 110 , DIsh was 119 and 110). Today - well , Dish is mostly on Eastern / Western Arcs, but they also have 118.