Having played 33/3, 30/6, 27/9, 21/18, 18/21, 18/18, 21/15, 15/24, 12/27 and so on, both mounted and on foot, melee, ranged and throwing, the overwhelming difference between agi and str build is this:
The player with the faster build dictates the terms of the engagement.
As such, str builds shine when the circumstances allow the player to circumvent this, for example mastery flags, siege defence and camping/ambushing.