You can't really go wrong with either set. Rex guided the New York Jets to the AFC Championship Game in each of his first two seasons, while Rob has turned around defenses in Oakland, Cleveland and Dallas since coaching the linebackers on two of the Patriots' Super Bowl-winning teams.
The Harbaughs are anything but slouches, though, and are arguably the superior gurus. John doesn't get much attention from the media, but that hasn't stopped him from leading the Baltimore Ravens to three playoff berths in as many seasons, with four postseason wins in that span, and firmly into Super Bowl contention in 2011.
Younger brother Jim, on the other hand, has proven to be a bit more abrasive than John (right, Jim Schwartz?), but may yet be the better coach. He's established a strong reputation as a miracle worker with quarterbacks, turning Rich Gannon into an NFL MVP at the age of 37, Andrew Luck into the top prospect in the NFL draft and now Alex Smith into the eighth-most efficient passer in the league. In the process, he's revived two long-dormant football programs in the Bay Area, most remarkably making an honest team out of the San Francisco 49ers without the benefit of a full offseason program.
The comparison isn't exactly a fair one, particularly since Rob has never been a head coach in the NFL. However, if we could somehow conduct a comparison of the two pairs, dividing coaching success by, say, the number of headlines made or media attention drawn, ironically enough, you'd probably find the Harbaughs' ratio much more top-heavy than that of the Ryans.
Either way, Buddy Ryan and Jack Harbaugh must be incredibly proud of their boys, all grown up and coaching like their dear old dads once did.