I'm also inclined to say there probably won't be all that much difference between the two days, except if the park closes at 7pm one day and 8pm the next, I'd choose the 8pm closing. (though that assumes you want to stay that late.)
If you aren't planning to buy G+, then later in the day AK is likely to be less busy. If you are planning to buy the multi-park G+, with the idea of booking morning attractions in MK then swapping to AK about the time you hop, the AK options might be somewhat limited. As another poster noted, a number of attractions close early.
Having visited WDW over holiday weeks many times over, I find them to be quite fun. We like the extra park hours. For us, if we planned to buy multi-park G+ that day, we'd tend to start at AK then hop to MK to max the number of G+ passes.
You also have the option of saving money by buying the AK-only version of G+ and book AK attractions starting in the morning. If you do that, then it will matter even less which day you hop.