|Gullyble Dwarf - Lvl 7 DM|
A double weapon counts the exact same as wielding a one-handed weapon and a light weapon when using both ends. There is no difference mechanically, ie you need the same feats to be effective and they have the exact same effects.
Some double weapons have slightly better damage than you'd have with a one-handed weapon and a light weapon like the orc double axe and dire flail which give you a d8 for both ends and I don't know that there's a light weapon that does a d8. Note: these weapons are usually exotic for most races.
If you have a double weapon someone only needs to disarm you once to make you weaponless. Sunder may work the same way here, I'm not sure though.
Dual wielding you could have more options, but typically people use the same weapon in both hands so they can take advantage of feats like weapon focus and improved critical.