I have looked long and hard for a guide to this. Maybe I was searching wrong but I cant seem to find anything. I know how dps is calculated for both, but what are the advantages and disadvantages to each of these strategies?
DUAL WIELD VS. 2HANDERS The Basics
If a 2 Hander is exactly 15% more DPS than two identical 1 Handers dual wielded, single target sustained DPS is identical. For example, a barbarian cleaves 4 times then hits Hammer of the Ancients on a single stationary target (remember what I said about simplifying assumptions for sanity?)
Barbarian has 100 strength. 1H weapon - 10 Damage, 1.0 Attack/sec - 10 DPS, with Dual wield bonus, becomes 1.15 attack/sec 4 Cleaves, 1 Hammer of the Ancients Total Dmg = 136 Total Time = 4.348 sec Net DPS = 31.3
2Hander - 15 Damage, 0.767 Attack/Sec - 11.5 DPS 4 Cleaves, 1 Hammer of the Ancients Total Dmg = 204 Total Time = 6.52 sec Net DPS = 31.3
Now, are 2Handers exactly 15% better than 1Handers? While leveling, this is hard to say, since item tiers unlock at weird levels. At the top end, the Arch Axe (top tier white 1H axe) has an average dps of 280.45. The Decapitator (top tier white 2H axe) has an average DPS of 566. Of course that doesn't mean jack squat since we're going to be using magic items anyways.
As far as I'm aware, 1H and 2H weapons have access to the same magical modifiers. So theoretically, dual wielding should give more stats.
Regarding itemization, items on a weapon that affect attack speed and damage directly, only apply to that weapon. I believe italso applies to any modifiers that are "On Hit". If the D2 system is still in place, this also includes life leech. This needs verification
Here is a brief summary:
One 2-handed weapon:
Personally I prefer dual wielding with Barbarians and Monks because of the faster resource generation. I feel this allows me to use my big skills more often, which I find more fun. But 2-handers do offer larger individual hits and crits, if you don't mind the longer swing timer and don't have fury problems.
If you plan on surviving inferno, it is widely accepted that a 1-hander and a shield are the way to go. Maybe this is different if you get godly gear or play a leap/stun hit and run strategy, which I just don't like as a barb. I have seen something like this be effective with leap attack followed by a heavy hit, which will be increased by a 2-handed weapon. I've also seen some glass cannon builds that can be effective, but again, most people don't want to play their barbs that way. These skills rely heavily on weapon throw and find themselves needing fury faster than they can get it. While this doesn't specifically answer your question at hand, I hope it gives you some options for planning for the future.