Are you asking about buying a 1 day ticket vs. 2 day ticket? I want to make sure I understand the question. So I will assume you are planning on spending only 1 day at MK and maybe 5-6 hours at EPCOT. Yes, that is what I am trying to decide.
I guess my first question is why are you staying on property if you plan to be off site so much or are you staying on property? Where else are you going? If going to Universal or SeaWorld, it would make more sense to stay at their hotels to get the benefits like "front of line" access. Have you been to EPCOT before? Do you like to visit the countries or are you looking for rides/attractions? We are only staying onsite for 2 nights. We will be doing Discovery Cove, Legoland and Seaworld. We have never been to any of the parks!
OK, on to the EPCOT question. For us, yes it would be worth spending the extra $$ for 5-6 hours in EPCOT. It is our favorite park and we enjoy touring and eating in WS. You can get most rides done in 3-4 hours if you use FP for Soarin' or TT and use stand by for the other. Remainder of attractions should be reasonable waits and you can see a lot in 5-6 hours and Illuminations is always a great show. I don't think my DS would be able to ride most of the rides do height. (We also have to be careful w/his neck as he has Down Syndrome and his neck is slightly unstable.)
A lot of the answers are based on assumptions as your original post I had some questions to clarify.
EDIT: Just saw the other post. $425 is a lot for any activity. We paid that for a day at Universal last time so we might still think it was worth it for us, but we LOVE Epcot as stated earlier. Tough decision. If you have the money available, I say go for it. If money is tight, I would say go to DTD and the resorts.