We have traveled both those 2 weeks the past two years. (14 day trips)
Memorial Day at MK was like our past Sept trips. Wonderful! Got soooo much done. (I think Epcot is hot spot on Memorial Day???)
The other days seem like typical summer days. Moderately busy but very manageable. I cannot say I saw a difference in the two weeks other than you will see a few more groups of field trips the week of Memorial Day. This was true this past year, but I think school was extended due to the hurricane and they filled the extra days with outings (SeaWorld had more field trips...but did see high schoolers at DHS).
Gay Days sees a slight bump in crowds the first Thurs-Sun after Memorial Day, but I think they have moved those plans to August for 2019.
So....of those two weeks....not really a huge difference between the two, but I would probably go with the first full week of June.
That being said, FWIW, I think July was the “sweet spot” for crowds this summer....and you get a few more later park hours and longer waterpark hours than you get in early June. But it is hotter in July.