Bracers of armor are good for characters that don't have armor proficiency or aren't allowed to wear metal armor. Or, if you don't intend to wear other armor (the bracers don't stack with traditional armor) they're a good way to get the ac boost without taking an armor check penalty or arcane spell failure %.
The buckler is better if you'll be wearing other armor but in addition to the -1 ac penalty and 5% chance of arcane spell failure there's also a -1 to two handed weapons or dual wielding (only on the hand that has the buckler)