DH surprised me with a trip to WDW for my Christmas gift. I leave 2/26!! Anyway, this trip is supposed to be all about me and practicing my photography. I'm still very much an amatuer and need lots of practice.

I only have a few days and EPCOT is always a must do for me, so I need to choose one other park.

I cannot decide between MK or AK. So, if this were you, which park would you choose and why?
I'd ask yourself these questions:
What areas of photography do you admire from others and want to learn to do better yourself?
What lenses would you want to get to know better? Do you intend to use a wider lens or a more telephoto lens?
Those two questions should really help decide between the two parks, MK and AK. Of course both parks offer some overlapping photography opportunities, such as candids, architecture, landscapes, rides, etc. But MK tends to lend itself more to various set pieces, characters, and period looks while AK tends to offer more safari-style and internationally flavored shots as well as more fine immersive details. If I was shooting wide, I'd say MK might hold a slight edge over AK - though the two main 'villages' of Anandapur and Harambe offer some very cool wide angle building shots and townscapes - MK overall is custom-fit for lots of wide angle shots and scenics, as well as motion photography, dark ride photography, and notably better night photography opportunities (except on the rare late-evening AK nights).
If I was shooting with telephoto, I'd pick AK in a heartbeat. In fact, I'd feel a bit restricted in MK with telephoto - it doesn't have as many opportunities to zoom on subjects, and many structures wouldn't allow you to stand far enough back with something like your 70-300 to even get the shot. You can of course shoot alot of close detail stuff, like masks, light fixtures, and flora closeups...but otherwise MK is not a place for telephoto to really shine. AK, on the other hand, especially when it comes to the animal walks and safari ride, absolutely require a telephoto, and can offer stunning telephoto opportunities for very long lenses. Moreso, the park has more open space and longer views across lakes and down rivers to other areas of the parks, so even scenics and landscapes can be taken with something as unwide as a 70-300.
I feel AK is the most immersive of all of Disney's parks as far as the landscaping, architecture, 'aging' and weathering, foliage, costumes, and for sweating a myriad of the smallest details down to small stickers on light posts, props resting unseen by most under bridges and walls, grafitti, missing bricks, rusty staples or nails, etc. So for a detail-oriented photographer, it is fantastic. And also to someone who likes to take closeup detail shots in such as way that it doesn't look like a shot from a theme park, but an actual location in the world.
Of course, Epcot has much of that detail and international styling too - which is why it's always been a favorite of mine; but you're already going there, so no need to sell you on it!