i live on the coast and the closer you get to the xmas and new year period the more traffic and police there will be on the GOR, this will mean a slower than normal trip.
i would say your better off with the earlier time cause its a nightmare down there over summer. also you should plan the return route a little better this time. we came back the back way last time and it was mad!! best drive i have had in a long time.
anyways just a thought. i will probably be there either way