I think offseason conversation is hard to come by so am not going to pile on over your choice of topic. I'm half-inclined to crunch some numbers but there are some problems with that. First, it's a lot of work. Second, the programs were in different shape when the coaches started their periods as HC in Nebraska. Third, the game has changed a lot in ten years and there's the little matter of a conference switch to boot (though it's fair to point out that Billy C. did get us to a CCG).
If I was to just look at the rankings of the performances their final teams put together, though, here's how they stack up nationally (after all, including bowl, games)...
OFFENSE
passing yards per game - 7 / 119 for Bill and 27 / 130 for Mike
rushing yards per game - 66 / 119 for Bill and 120 / 130 for Mike
points per game - 28 / 119 for Bill and 84 / 130 for Mike
total yards per game - 9 / 119 for Bill and 87 / 130 for Mike
DEFENSE
passing yards per game - 84 / 119 for Bill and 60 / 130 for Mike
rushing yards per game - 116 / 119 for Bill and 115 / 130 for Mike
points per game - 114 / 119 for Bill and 116 / 130 for Mike
total yards per game - 112 / 119 for Bill and 101 / 130 for Mike
Per this quick and dirty comparison of the coaches' final seasons, it's readily apparent that both defenses fielded were pretty sorry, in terms of their performance, with the Riley bunch getting a slight edge. There's really no comparison offensively, outside the running game (and even then it's a clear winner, relatively speaking, for BC).
To sum this up, Bill might fairly quip that coaching at Nebraska was "too technical" for Mike Riley.