Unless I'm confused, you don't currently have to mark a ticket, even a non-park-hopper, as being for a specific park. (Confusingly, you do have to make a park reservation, independently of your ticket.) Here's an idea: halfway through your trip, decide what you want more of! You can change park reservations at the last minute, as long as there's availability. So whatever you decide on, you might be able to change that decision on the fly, if there's something you didn't get or something you want more of.
But it's a tricky question anyway. It's easy to spend a lot of time in Epcot, just because there's a lot to see and it's spread out. It's easy to spend a lot of time in MK and, increasingly, HS because the rides are popular and you might wind up waiting a while. And although there's comparatively less to do, AK is a beautiful park to just hang out in (especially at night). So I'd say to double up HS for the sake of the high-demand rides, or MK since they just plain have more stuff, but you'll have AH which should take care of those thoroughly. I'm probably in a minority, but I might take that extra day at AK. But Epcot is probably a smart choice too.
At any rate, with this many days, you've left yourself a comfortable margin to go to the parks in the mornings when crowds are lower, leave in the afternoon and relax at the hotel, and then come back for lower crowds again in the evening (especially for the AH events!).
Is this a first visit? Are there kids in the party?
