This is just my opinion, but... Any day I am going to MK, I want FP+ for MK. All day, part day, morning, afternoon, evening, I would rather have FP+ in MK than any other park.
Now I will admit... I got totally hooked on 7DMT in April this year, and FP+ is really important for that. My favorite things to do at AK are the Safari, the shows, the animal trails, so I really can do what I want at AK without FP+ if necessary.
I think the most important factor is, what are your top priority attractions at each park, and which of those attractions are most dependent on having FP+.