Cool, thanks.
Poly rooms are larger than YC and in some ways are more "livable" with four people. But the new YC rooms (which I really like) use space very well and are passable. Just not as big as Poly.
Poly's bathrooms: Shower/tub, sinks, and toilet are all in one space with a door separating the room and bathroom area.
YC's bathrooms: Shower and toilet are in one space with a door, sinks are outside of that space (with a sliding door that can close the space off from the room). So, someone can be drying their hair while someone else has some privacy.
I mention this because the fact that the toilet is in the one bathroom area at Poly is sometimes a consideration (read, gripe) for some. Depends on how you travel / if that matters to you though.
I agree with
@DisneyWishes14 in that Poly has more of a "wow" factor, for us. I never tire of walking in that lobby, smelling the wonderful scent. Some people feel the lobby is noisy and chaotic (esp in the evenings with Ohana being such a big draw) which I get, but to me it's exciting. Then when you walk out the 'back' door and have the castle in site and have the buzz/happiness of the Lava Pool and other things around the Grand Ceremonial House, it all combines to create a wonderful and unique place. Looking out over the 7 Seas Lagoon towards GF, MK, and CR never gets old to me. It's pure 1971 "vacation kingdom" Disney and is one of our absolute favorite places in all of WDW.
YC is a bit more refined and sophisticated. More sleek, more stylish, more subdued. Less wow factor, although a room facing Crescent Lake is mighty nice (will depend on your view category of course). Storm Along Bay (SAB) is better suited for teenagers, to some extent. If swimming / a lot of resort pool time is on your agenda, then I'd weigh this somewhat heavily. One negative with Poly is the Lava Pool can sometimes be crowded to the point of being annoying at peak times, although the Oasis Pool helps a bit (and is really nice). SAB is probably the biggest "wow factor" element of YC to us.
Your park plan (MK & Epcot) tends to make me think Poly more than YC. Monorail and boat access to MK; short walk to the TTC for Epcot access. We hit Epcot a lot when staying at Poly. Sure, not quite as easy as the walk-able access from YC, but it's the next best thing. An Epcot arrival via monorail is kind of a ride in and of itself with the great views of the park.
Other Poly highlights include things like nightly Electrical Water Pageant viewing and nightly MK fireworks viewing from the beach with audio piped in. Your plans may make both of these non-factors though. My kids are younger so we're sometimes back at the resort vs in the parks around fireworks time, so that's how we get our fireworks fix in. Might not be a consideration given your ages and if you're park commandos.
YC was the last deluxe for us to try and I had pretty much always dismissed it when planning previously - just never really called to me. We stayed there for the first time earlier this year and have already been back once since then. It's a warm and inviting resort.
If you're finicky about hotel rooms, Poly's resort rooms were last renovated 2013ish which is getting a bit long in hotel years (you may see it in overall fit/finish in some rooms). YC rooms were totally re-done last year and are very sleek/nice (I like it, but some object to the hardwood flooring).
Sorry - long post. Like any WDW resort, there are pros and cons. Locational advantages and disadvantages. Different room view categories will give you different experiences.
Since I really like both, but for different reasons - my advice above is sort of a mixed bag!
The good thing is, I don't think you can make a "bad" choice given what you describe. Best of luck in your planning, whatever you decide.
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