Dualwield, better weapons and more possible affixes. But Twohanders are cool, you get a talking sword ffs.
I was more thinking in the line of Halberds, but my inner powergamer whispers me too that Dualwielding is stronger.
Currently playing a Kensai/Mage myself, and I can only agree with Roller. But if you're thinking from a roleplaying perspective it makes sense too; A Fighter/Mage would be a sort of specialist in my eyes, combining magic with weapons, foregoing armor for pure offensive power; I'd leave the twohanders to the berserkers and barbarians. ^^ But ofc, just my personal opinion. There are some completely badass twohanders in the game, just choose what fits your style best. Ofcourse, as a Fighter/Mage (multiclass) you might be able to specialize in BOTH twohanders and dualwield (Due to only 2 stars allowed), if you'd like to.