You can't bring Luffy punching Garp into this. That fight was thrown by Garp, who took the blow without trying to stop it, and after he fell Sengoku called him out, saying "Even you are still a father, Garp". If it was a full-on fight, not just a single punch, then that would be a valid argument.
I'm inclined to think that it would be a pretty close fight. Garp has experience and potentially superior Haki, as has been claimed earlier in this thread. Akainu has a very powerful Logia that can lay waste to the battlefield, and is arguably in his prime, or just past it. If they were both as combat-ready as they could be, health-wise, I'd wager it could go either way, objectively.