To me the answer hinges on what aspect of EP is more important to you - riding the FW rides, or enjoying the cultural aspects of WS. WS is my favorite place in WDW, and PM EMH at EP is one of my all-time favorite things to do. In my own experience, I have not noticed PM EMH at EP to have a significant impact on RD crowds.
On a high crows day, you’ll want to be at the gates at LEAST an hour before RD and “walk with purpose” to either TT or Soarin (assuming you have FPPs for the other one). I prefer Soarin’ because as soon as I finish I can go over to Living with the Land before the lines build too much for it - but it’s really a personal preference. If you are interested in Mission SPACE, I recommend getting FPPs for it as well. I usually get SE as my 3rd FPP because it’s one I don’t want to miss. Seas, Energy, and Imagination rarely have really long lines (and if the line for the Nemo ride is long, you can skip it and enter the pavilion through the exit and enjoy the rest of the Seas exhibits).
If you have PHs, to me the answer is easy - Do RD at EP and hit the FW attractions hard, using lower RD crowds and FPPs to get as much done as possible. Consider having lunch in WS. If the crowds/lines seem “reasonable” stay at EP. If it seems too crowded, hop to another park (maybe pick up a 4th/5th FPP while you’re there) and spend the afternoon there. Then, hop back to EP for dinner and some EMH touring.
If you don’t have PHs I’d still go to EP for RD, and if the crowds are too much mid-day, leave the park and do some resort touring (GF, AKL, and WL are all especially pretty at Christmas time), and then hop back for dinner and WS touring.
In any case, do NOT miss IllumiNations with the Holiday finale.