I've just started a 3 person party, I need dual classing advice Options

[Klorox]

I'm a big fan of small parties that can cover all of the roles needed. My newest venture into this is a party consisting of Anomen (fighting/divine spellcasting), Imoen (thieving, main arcane caster), and a Human Berserker > Mage (fighting spellcaster).

Let me just say that IMO there is two ways an arcane spellcaster can go: You can be an all-out wizard type (Imoen in this party) or a fighter who uses arcane spellcasting to augment their warrior abilities (this is what I intend to do with my Berserker > Mage).

I just don't know when to dual class this character. My choices are between levels 9 and 13. There are pros and cons for each.

If I dual at level 9 I'll have :
3 Rages per day (I think this is enough)
I'll become a "complete character" (Berserker abilities back after dualclassing) at 500,000 XP

If I dual at level 13, I'll have:
4 Rages per day
1/2 extra Attack per Round
I'll become a "complete character" at 2,750,000 XP

I know dual classing at level 13 will end up with a more powerful character in the end, but is it worth the 1,250,000 XP?

Let me know what your experiences tell you. TIA