Please keep topic titles specific: see my edit.
That said, as pointed out above, the differences vary from cable system to cable system. It's almost a sure bet that if the quality you're seeing is bad enough for you to question it, DirecTV will be better. Were it not, this board would be full of complaints like it used to be in the days before D* expanded into the Ka band. Since then, HD PQ has improved.
Set your home DVR to record a particular channel. Go to a satellite dealer, friend's house or other place that subscribes to D* and watch that channel. Go home and watch your recording.
Naturally, it's better if you can A/B it, but there are few geeks such as me, who have both.