Difference being that Power Attack doesn't affect your die rolls down the line. Monkey Grip does. I would only take a penalty at this level, when hitting is so lethal, if it boosts you in other ways. For instance, I take a -2 to hit with both Wolf Fang Strikes, but hey, hitting twice and adding 1d6 to each attack is a bargain. Take Exotic Weapon Proficiency (Heavy Weapon). It's Monkey Grip but without the penalty.
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Sburbopolis, Homestuck RPG: http://www.gamefaqs.com/boards/8-gamefaqs-contests/60981373