For what it is worth. During our first trip to AK, we were talking about the heat to a CM and they said that on average AK was like 6 or 7 degrees hotter than the other parks.
That's complete nonsense. The difference between parks can't be more than (maybe) a single degree. If two areas that close together were 6 degrees different, without a significant corresponding change in altitude, there would be high winds and a storm front right there in between the two parks all the time.
Ditto for relative humidity. There isn't a significant difference between Animal Kingdom and any of the other three parks, because the controlling factor is the air over Orlando, not whether AK has more vegetation.
In any case, I have hopped between the parks on the same days, sometimes as many as three in one day. When it's hot out, they're ALL hot. When it's humid, they're ALL humid.
Practically speaking, Epcot might be a bit warmer in the afternoon and evening, because as the air temperature starts to drop, all that concrete is still radiating heat away. This happens with all the parks, since there is concrete in all of them. But there's significantly more concrete at Epcot, so it's possible that it's a degree or two warmer at night than the others.
However, as others have pointed out, there are a lot more air-conditioned 'stops' in Epcot than in AK for people to get a brief respite. Innoventions east and west, the Land, the Seas, various shops, indoor rides and movies around world showcase, etc. So people probably do feel hotter or sweat more at AK than Epcot, because they're not getting as much opportunity to cool off.
But considering the actual weather at the four parks, it isn't really much different.
David