As they say, a system is only as good as its weakest link. In this case, it's using the RF out of the set-top-box and the RF in of the AIW. That is the absolute worst of all mechanisms. You could be right next door to the studio and it would still look crappy. Are you even getting stereo sound? I don't know anything about the AIW or your set top box, but many don't pass stereo sound through the RF link.
First step...
Separate the audio and video. If the box has stereo outputs (red and white RCA jacks), use them to connect to your sound card. If you want the ultimate, most digital boxes have digital audio outputs, both optical and coax. If your sounds card has digital inputs, give that a try.
Second step...
Use the best video transfer method common to the 2 devices. RF is the worst, Composite is better (single yellow RCA jack), S-Video is even better (a single DIN type jack) and Component is the best (3 RCA jacks, 1 red, 1 blue, 1 green). Even though I listed these in order of technical superiority, that doesn't mean that they will look that way to you. Use the one that looks the best.
All other things being equal (they frequently aren't), digital cable and digital satellite should be the same from a quality perspective. From a reliability perspective, my neighbors who are AT&T Digital Cable subscribers have had more outages in the past month than I've had in the last 7 years as a DirecTV customer. Any weather event serious enough to knock out the satellite either knocks out the cable as well (for a much longer term) or has you scrounging sandbags in which case you'll never notice.