really down to price, powder coating will be better and harder wearing but cost twice as much. all aluminium wheels will corrode(or really oxidise) if subjected to the elelments so if paint chips or powder coat chips then the metal underneath will oxidise, but you wont see it really until it gets so bad that it lifts the paint.
this is the reason poeple say not to put clear laquer on polished rims, as once they chip water gets in and it is visisble as the surrounding paint is clear.
my wheels have only been painted in normal water based paint (the one i had as i resprayed the car) and they are four years old with no issues, but we dont have as much rain as you do. bottom line is paint is fine but you must look after the things as you would any painted panel, i.e. clean and wax them, fix and touch up stone chips etc etc
