What day are you going? Depending on when you are going, AK closes at 8 p.m. I don't know if I would waste a park day on my tickets for 4 hours.
In terms of crowds, usually the park that has extra magic hours will be the busiest park. That can be due to several things. Those people staying on property that only have one park per day tickets can only go to AK if they want to experience that park. Also, people who are park hopping that day might go to AK later and take advantage of a less crowded park in the morning that doesn't have the extra magic hours.
AK seems to be crowded ever since Pandora opened.
I actually have the same plan as you on my upcoming trip of going to AK first (it's a tradition of mine), and I'm going to MK the next day. The only difference is AK doesn't have extra magic hours on my first day.