You are correct that strength gives more attack power than agility, point for point. But agility is generally considered to be as good or better a stat because in addition to attack power it gives you crit, dodge, and even armor. So it's easy...focus on both. Strength to boost your AP and Agility to increase AP in conjunction with other key combat stats.
Specifically, a point of strength gives you 2 points of attack power, multiplied by 10% with the Heart of the Wild equals 2.2 AP per strength. A point of Agility gives you 1 point of attack power, multiplied by HotW to equal 1.1 AP per Agility. But Agility also increases your crit by 1% for every 25 points, your dodge by 1% for every 14.7 points, and your armor by 2 for every point.
All actual knowledge compliments of
http://wowwiki.com/Agility#Agility