I've seen lots of comments on this thread about getting an armguard. I wear one religously, but usually just to hold my shirt tight, and for that occassionaly slap. I have three longbows of which none slap my arm. I couldn't imagine accepting stringslap. The noise alone of the string hitting the armguard would make me switch bows.
I think the input about bow arm, grip, and form should be considered more than the armguard.