Swashbucklers: Agile Shortsword w / piranha strike or Katana w / slashing grace

I'm trying to decide which build is better for a level 7 swashbuckler, i like the idea of piranha strike cause i'm of the mind set that higher damage equals better, but all the forum posts i see promote using slashing/fencers grace.
so to break it down...

Build A: Piranha strike with an agile short sword allowing for more flat damage at a penelty to hit, with an inferior weapon 1d6 19-20

or

Build B: Katana with slashing grace, less damage, higher attack and crit but requires exotic weapon prof

or

Build C: Rapier with fencers grace, same as build B except lower die damage and doesn't require exotic weapon prof

I feel like im sold on build A but does any one have some insight for me on the subject that might influence my decision?

I think we're gonna need to see your build in its entirety before we can really comment. I will say that, in my opinion, the difference between Slashing/Fencing Grace and an Agile weapon is like the difference between Keen and Improved Critical, can you better spare a feat or cash?

At 7th level the agile shortsword is a decent option, though I'd personally prefer a weapon with a higher threat range on a swashbuckler.

Part of the reason why people promote Slashing/Fencer's Grace is that these feats can come online very early, while most characters won't be able to afford an agile weapon until the low-mid levels. Playing levels 1-6 with no ability modifier on your damage roll is obviously less than ideal.

As the build isn't complete I don't have much to give you,

Swashbuckler level 7
Race Azata-blooded Assimar
Str 10
Dex 18
con 14
Int 12
Wis 11
Cha 16

my feat selection varies depending on the weapon I choose, but currently its built for an agile short sword

Feats: Weapon focus, Shield Focus, Piranha Strike, Combat reflexes, Quick draw

This is a solo game so the dm has suggested I take some feats like iron will to cover my save bases.

Yeah, I think I'd go with an agile weapon and Piranha Strike over Fencing/Slashing Grace since you're starting at 7.

You can use a larger weapon with Piranha strike. Effortless lace drops the effort to use a rapier by one so you could make that rapier light. Take Fencers grace. Dex to damage and piranha strike.

Effortless lace? explain this magic of which you speak...

Did you roll those scores or are you using point buy? If possible I'd consider switching Int and Wis to help shore up your will save. Iron Will is a good idea. If you plan on boosting your charisma further you can also consider Steadfast Personality, Divine Protection's less nutty but still powerful cousin.

I used a stat array

16, 14, 13, 12, 11, 10

Effortless lace? explain this magic of which you speak...

It's from the giant hunters book.

http://paizo.com/threads/rzs2rtj2?PistolWhip-ability-does-it-work-with-Weap on#9

Ah I'm sold. adding a little bit of lace to my weapon should add just the level of flamboyancy required to be a swashbuckler ^_^

Did you roll those scores or are you using point buy? If possible I'd consider switching Int and Wis to help shore up your will save. Iron Will is a good idea. If you plan on boosting your charisma further you can also consider Steadfast Personality, Divine Protection's less nutty but still powerful cousin.

Would that 'Steadfast Personality' feat stack with the Paladin's 'Divine Grace' class feature that already allows him to add his Charisma mod to all saves?

or you could ask your DM if you can use the 3rd party feat self delusion which just replaces wis with Cha for will saves

Yes.
Relevant FAQ.

"However, you can still add, for instance “a deflection bonus equal to your Charisma modifier” and your Charisma modifier."

Steadfast Personality adds an insight bonus, so it stacks with Divine Grace or Divine Protection.