Ultimate Magus: Which spontaneous class to use? [3.5]

[Ilmryn]

Assuming I go Wizard on the prepared side, should I take Sorcerer on the spontaneous side for versatility, or Duskblade etc. for insane amounts of low-level spell slots, and thus lots of Augmented Casting? Duskblade's buffs would also free quite a few Wizard slots that would otherwise be spent on blasts or buffs.

The usual recommendation is Beguiler on the spontaneous side for SAD, and utility spells. If you have access to the Dragon Compendium, I would recommend going Shi'ar on the prepared side, and then take any cha-based spontaneous caster on the spontaneous side.