Advantages of the 430 EX II over the 320 EX:
- Automatic Zoonmreflektor which covers 14-105mm, compared with 24 and 50mm manual
- Discreet red AF illuminator towards nervigem flash continuous fire
- Flash release button eg for hiking flashes (the 320's really not)
- Manual power setting (useful for remote triggering with external radio receivers)
- Backlit LCD display instead of two indicator light.
Conclusion: for the ambitious amateur or professional, the little more expensive 430 EX II simply more versatile device.