I'd go with Replicate Casting. There's something cool about being able to mimic spells. I'd use it mostly to mimic buff spells I can't cast.
Also, take a look at 2 PrCs for a Sorcerer from Age of Mortals:
1 - Academy Sorcerer. On a Snowcasting Sorcerer, that's +4 DC, +4 Spell Penetration, +8 to Saves. vs. [Cold] and 3 Metamagic Feats over 10 levels. Oh, and you can Aid Another to add a +7 bonus to some skills to allies. And of course, UMD is a Class Skill. Not quite Incantatrix, but it's close.
2 - War Mage. Crappy feats to enter, but at low levels Combat Casting isn't that bad, and you plan to enter early, don't you? Oh, and you get 2 Metamagic Feats for free over 5 levels. And CHA to AC to 3 Allies. And +3 Damage per Die. And -10% ASF.
If you can cheese your way into it, the Citadel Mystic is beyond worthy.
Benefits: Bonus Domain, CHA to 2 Saves, 4 spells that are permanently either Extended of Empowered. As a Favored Soul, I'd want it bad.
All 3 are full casting PrCs!