I think there is only a slight imbalance between str and agi..
Just nerf the free wpf you get per level and transfer it to weaponmaster and we are fine..
Agi is a fun way to play, as you can pull off a lot of tricks, often decide your battles, and mistakes punish you hard. The heavy armor str char only requires the enemy to make 1 mistake and bam. Often you'll get free kills by enemies either not able defend or not looking your way.
STR is for the chaos of battle and big ass long weapons. Works wonders with teamwork. You can afford to do mistakes, and still dish out heavy as hell damage.
AGI is the soloist playstyle who can create 1vs1 situations. Be sure u can block very well though, and stay cool facing multiple opponents.
Looking at AGI vs STR pitted against each other in a melee, I think they are fairly balanced. Throw in ranged and cav, and the balance favours STR chars.