Get the 580EX2 if you need more power, wants faster recharging, easier to use rear control. It can be used as either master or slave in a multiple flash setup.
Get the 430EX for the lower price, lighter weight. Can only be used as slave in a multiple flash setup (but since you'll need to get more flashes for a multiple flash setup anyway, the extra flash you get can be a 580EX2).