It depends on what you want to be better at.
TWF has a higher hit rate, especially with Vistani Knife Fighter's Double Dagger (or monk dip with Shintao's Deft Strikes - check the old forums for wizard 18 / monk 2 build using handwraps), which means more chances for proc-on-hit effects like Imbue damage, vampire heals, etc. But you miss out on the dual-wield goodies which Tempest gets like Dual Perfection (full offhand damage), Dance of Death, etc. Unless you do something heavily multiclassed, in which case you miss out on higher-level wizard abilities. Plus you need DEX 17 for ITWF/GTWF so it's more MAD than SWF which only requires 7 ranks of Balance.
SWF has a higher damage modifier which means more damage from Cleaves or Whirlwind. Also you presumably have a caster orb in your offhand, which helps out with gearing a melee/caster hybrid like Eldritch Knight.
Unless you're already set on dual-wielding, I would probably just stick with SWF. Easier to gear, doesn't require a DEX tome, and more damage from your Eldritch Strike/Tempest attacks. If you use bastard swords or dwarven axes, you'll also get a little Strikethrough on regular attacks which can be boosted further with epic abilities.