I'd go with DMC: Hurricane because Unsolvable Riddle is more of a PvE trinket. Here's some info on both.
Available from Tol Barad for 125 commendations, this trinket is instantly recognizable as a PvE hunter trinket, with a giant on-use agility boost. If you use it every chance you possibly can, it has a 16.67% uptime -- a very common uptime for Cataclysm trinkets. This makes the average agility provided 267.5.
While the Unsolvable Riddle is clearly attractive compared to heroic trinkets, it falls behind the curve once we compare it to raid-level trinkets.
Darkmoon Card: Hurricane
DMC: Hurricane is the subject of much debate in the hunter world. We're used to comparing stats, but the big thing with Hurricane isn't stats; it provides a flat extra attack. Here are the key bits of info, based on the best information we now have:
The proc chance is about 5%, with no internal cooldown. It can proc as often as chance favors you.
It will not proc off of your pet's attack or DoTs, including Serpent Sting, Black Arrow, Explosive Shot, or Piercing Shots.
It uses the spell crit multiplier and the spell hit chance. So even if you're hit-capped, this special extra damage can actually miss.
The damage is increased by survival's mastery.
The biggest complication for this shot, of course, is SV's mastery boosting its damage, but in general while this trinket is good, there are better ones available for hunters. Of the specs, MM gets the most benefit from the trinket, since they fire the most shots that could trigger a proc, including Wild Quiver. Neither SV nor BM can proc the effect off their signature shot. Those fast-attacking, dual-wielding classes are the ones who really want this trinket.
Credit goes to Joystiq for the quoted information above.