The mains charger provides more power than the USB... USB is just a trickle charge.
btw, with Lithium batteries (such as with the Bold and almost all other new toys these days) - the best way to look after the battery is to make sure that when you start charging, you keep charging until the battery is fully charged. Sometimes it can not be helped, but try not to interrupt the charging unless you can help it. Just my $.02 worth!