A few more balancing factors for TWF (double weapons in particular):
* Weapon Specialization applies to both ends of the weapon...this is like an extra 2 damage above and beyond what you would get with a 2-handed weapon.
* Double the weapons, double the frost, flaming, and icy enchantments...by using a double weapon you can have more capacity for enchantments.
* Double weapons can give you higher maximum damage than 2HF. This can be crucial in cases where an enemy has to be killed quickly in one round. With DWF, you can "get lucky" and turn the tide of the battle where a 2H Fighter might not have enough damage capacity to do so. Sure, on average the damage of the two styles will be comparable...but sometimes the average doesn't matter as much as the maximum.
* Double weapons can be more AC-friendly. Use a buckler on one hand, or have both weapons as Defenders, for example. Off-Hand Parry and Twin Sword Style (from FRCS) can further boost DW Fighters' defense.
* Masters of the Wild has the Greater Two-Weapon Fighting feat and the Tempest prestige class. Both significantly boost the power of TWF.
Overall, I think the two styles are basically balanced, with TWF being very slightly weaker and more magic-dependent overall.
* Weapon Specialization applies to both ends of the weapon...this is like an extra 2 damage above and beyond what you would get with a 2-handed weapon.
* Double the weapons, double the frost, flaming, and icy enchantments...by using a double weapon you can have more capacity for enchantments.
* Double weapons can give you higher maximum damage than 2HF. This can be crucial in cases where an enemy has to be killed quickly in one round. With DWF, you can "get lucky" and turn the tide of the battle where a 2H Fighter might not have enough damage capacity to do so. Sure, on average the damage of the two styles will be comparable...but sometimes the average doesn't matter as much as the maximum.
* Double weapons can be more AC-friendly. Use a buckler on one hand, or have both weapons as Defenders, for example. Off-Hand Parry and Twin Sword Style (from FRCS) can further boost DW Fighters' defense.
* Masters of the Wild has the Greater Two-Weapon Fighting feat and the Tempest prestige class. Both significantly boost the power of TWF.
Overall, I think the two styles are basically balanced, with TWF being very slightly weaker and more magic-dependent overall.