17Clr/2Mnk/1Ftr or 18Clr/2Mnk TWF Rapiers

[Rikien]

I posted this in the class builds forums yesterday, but I remade it so many times that I decided to re-post it over here in its latest version.

Going for survivability rather than DPS, but still being able to heal in groups.

Rikieum Aers

17 Clr / 2 Mnk / 1 Ftr Drow Elf

Starting stats:
14 STR
16 DEX +2 enhancements, +1 tome
12 CON
10 INT
16 WIS +5 levels +3 enhancements
10 CHA

Skills: pump concentration and balance.

1 - Toughness
1m - Two Weapon Fighting
3 - Extend Spell
5m - Power Attack
6 - Empower Healing Spell
9 - Maximize Spell
9f - Improved Two Weapon Fighting
12 - Improved Critical: Piercing Weapons
15 - Greater Two Weapon Fighting
18 - Quicken Spell

Going Radiant Servant. Using rapiers for the most part.

Should I switch Greater Two Weapon Fighting or Improved Critical: Piercing Weapons for some spell penetration, a metamagic feat or maybe weapon finesse?

Any comments welcome.