The cage length effects the tooth capacity of your drivetrain, long cage having the largest capacity, basically dictating which gear ratios you can select. Short cage deraillers are a little lighter (barely), and I should think are marginally less likely to strike rocks etc, but I've always used long cage so I don't know which is actually better. As for whether this years sram stuff is better than last years, I have no idea, haven't used it, sorry.
I picked up an x-9 medium cage der. Supposed crisper shifts, less chain slap, and some other stuff. Had it for about 4 months (?), but not that much riding (winter). Maybe 15-20 rides or so.
From what I can tell it seems shifts nicely compared to my old x-7 that I bent the crap out of, at least from what I remember. The shorter cage is also nice b/c not as close to the ground.....very slight diff, but who knows how that extra bit might help out.
Big/small gear is a no-no as the der will fold back in on itself, but that much chain cross isn't good anyway. However, i've gotten away with 'almost' the big small combo (by accident of course).