[SirAlvarex]

I'm no expert, but from what I've gathered, the general rules are:

Pro 2h:
1. Fewer attacks mean fewer riposte's at you
2. Higher damage riposte
3. Damage bonus on 2h with slower than 40 delay is bigger than standard weapon bonus
4. Single double-attack check doubles all potential damage output, instead of only half of it

Pro Dual Wield:
1. More even damage due to a lower spread
2. Typically lighter weight weapons
3. Can obtain two weapons with a combined ratio higher than 1:1

But really, the uber damage bonus for 2h weapons is so insane, and coupled with the fact that the offhand doesn't get the bonus + you won't hit/double attack with it 100% of the time means 2h weapons win in a landslide.