The 430 has custom functions, and you can set 2nd curtain flash on the actual flash. Also has an LCD on the back which is nicer than what the 420 has.
The button for swiveling the flash is also in a more comfortable and convinient position (I know it dosen't sound important, but if you do it alot then you will embrace this feature).
You can also set flash exposure on the 430 directly.
Basically you just get more control on the 430.