Ive been wondering the same exact question. While leveling however, it seems to me that it makes little difference,.the dps difference between the two choices (as a windwalker monk) seem small to me in gameplay. On a Frost DK, I absolutely roll with dual wield, and the difference for frost dk's is in fact noticeable,..but for monks (windwalker),,,seems to me that the differences are small enough that it becomes more a question of "visual preference" (i think dual wield looks cool,,and the graphic display of the speed of your attacks visually, can be frustrating to newbie opponents in pvp bg's,.it just looks like your smashing their face twice as fast,..dps wise however,.the 2H wpn might do slightly more dmg overall,,,but the value seems small enough to neglect if u just like the appearance to dual wield)
Also,.its nice to be able to add two enchants to your weapons (1 each),.instead of a single enchant on a single 2H wpn.
However,..by the time you level-cap,.you may prefer the single 2H wpn.
That being said,..if you enjoy dual wield,,continue to do so until you level cap. Once you do that,.you'll be remodeling your toon anyhow,.you can revisit this question at that point.
Until then,.my vote is DUAL WIELD