Sheik v. Falco's pretty even. Both got ridiculous damage rackers. Falco can wall out sheiks ftilt pretty well onstage w/ jab, sheik cannot directly challenge jab outside of aerials/getting jab in first as well. Both of them edeguard each other well (well, sheik edgeguards harder imo, but her recovery's much worse), needles keep falco's camp game in check to the point of making it hard to outright camp her if he's behind in the lead.
Then there's the fact that Falco loses just as badly to Pikachu and ICs, and loses to Sheik almost as badly for the same reason.
http://www.youtube.com/watch?v=9CWHqwjJa70 is a fun video to watch.
You forgot that in that set, BluB won (although tbh, Biggad should've won round 3, everyone sitting there when he dtilted at high percents said he dsmashed but got a bad input, and he tripped, which was silly).
@Seagull: What happened to DIing the spike onto the stage? Mind you she probably still has to take a free smash/aerial after unless the player's super pro and can tech that kind of thing consistently, but it's better than having to eat tons of damage trying to recover.
tl;dr
Whoever's offstage: loses lots
If sheik is at 0-30%: Falco's going to win in terms of making the opponent's options implicitly limited
If Falco is at 20-40%: Sheik is going to win from the same principle as the former mentioned
Whoever's got a grab: imo wins, both set up for aerial kills pretty well
Very momentum based, so even getting the lead won't make things guaranteed for either party.