We always choose MK on Dec. 30th and Epcot on Dec. 31st.
I would not worry about capacity. There is always a possibility of phased closures during parts of the day at MK, but they are few and far between and they generally happen late morning and are lifted a few hours later. If you’re staying on property and/or have an ADR, you should be fine. I would not recommend getting to a park early (especially New Years Eve) or you’ll burn out by the evening. Sleep in, swim, have a nice lunch at your resort, and head over later.
We usually head to Epcot late afternoon/early evening on New Year’s Eve. We’ve never had any issues with buses or being stuck in traffic this time of year going from our resort to the parks. Some areas have dedicated bus lanes to bypass traffic. Our favourite Christmas/New Years resort to stay at is POFQ. It is my favourite time of year to be at DW.