I've had both (currently DTV). Sling TV used to have a lot of issues but it's pretty solid now. My biggest issue with Sling is the lack of content and relatively low streaming quality (compared to others). DirecTV Now has awesome quality and a great lineup, but man stability is lousy right now. I went weeks with maybe one or two errors a day, but the last week it's been pretty bad on my end.
I also had Vue for a while. It's probably the best in terms of features and stability, but a little pricey (especially if you are "lucky" enough to get local channels) and the content has been a little sparse since they dropped Viacom.
I assume DTV Now's quality is 1080p. You're saying Sling's is lower than that ? How bad is it ?
I signed up recently for DTV now and just started really using like yesterday and it was nothing but freeze after freeze yesterday pretty much ever other minute and alot of unable to play errors today. I really hope they fix this within the next couple of months.