In part it depends on the AC of the monsters you're fighting. If they have high AC, you're best off boosting your single attack, but if you can land many hits, you get more damage. If you want the exact expected outcome, you can Google it. Multiple attacks are also better against groups of monsters. It will also depend on your party: is there a bard to boost your attack rolls? a necromancer to debuff your adversaries? Are you the main tank, or can you afford to maneuver a little (which swordsages do very well) while someone else babysits the casters?
Generally, two-handing will give you more damage output than multiple attacks, especially if you take advantage of the 1.5 multiplier on Power Attack. In my experience, martial characters end up having ridiculously high modifiers (in the +20 range by level 12) so, I'd not worry about having a medium BAB as long as you have decent Strength and invest in a magic sword.
Multiple basic attacks really stand out when you have something else giving you an extra effect (like a vorpal sword's killing chance) or elemental damage. A lot of magic weapons are activated on a standard attack, so find something good and exploit it.