Ounce for ounce, inch for inch forged is a touch stronger but billet can be machined to be stronger by leaving thicker material at stress points. Most of it boils down to aesthetics, billet will usually have more artistic lines, integral trigger guards, bigger openings in the magwell, stuff like that. The integral trigger guard is the biggest plus in my book, saves the stress you have worrying about breaking off an ear when installing the roll pin for the trigger guard, doubt it happens very often but always stresses me until the pin is in.