check to see what's open during both for the day you want to go. There was next to nothing open at night in Sept, but others have posted differently for the times they were there.
The am hours are great! we arrive at park opening, do safarrie, dinosaur, primeviel whirl, triceratops spin, tough to be a bug, kali river rapids then boneyard all by lunch time in Jan!!1 Then we eat and see Lion King and Tarzan if we're up to it!!
A.M is better, you get there before everyone else and by the time the park opens you've done almost everything. I agree do the big rides first because those are going to have long line in the afternoon. Once we did EMH at animal kingdom and by 9:00 we were done and going to another park.
Since AK is my favorite park, I'd have to say I love both, but I prefer evening in some ways. Hardly anyone's there, so it's easier to just stroll around and take in all the little details of the park.