Oh, goodness, this is so subjective.
There are times during my 20s/30s when I loved Poly. Easy access to MK, on the monorail loop for dining and drinking at other resorts, and a quick walk to the TCC to get to Epcot.
There are other times during my 20s/30s when I loved Epcot area resorts. Amazing access to Epcot, easy access to DHS, and great access to dining and drinking at Epcot's world showcase and at boardwalk area establishments.
Personally, I find both Poly and BC to be busy resorts. Poly is often crawling with visitors for general interest, 'Ohana, Luau Cove and now Trader Sam's. It's most apparent in the GCH, but on all stays there (in both high and low season), have noticed heavy traffic on the grounds after Luau performances. BC's lobby is also busy, thanks in large part to Cape May. Boardwalk area just outside BC is also busy with traffic to Epcot/general boardwalk area/Hurricane Hanna's. However, rooms in both are large and lovely and generally quiet. I just wouldn't call either resort 'relaxing' based on my experience. Both can be relaxing, but both can also be hives of activity.
I love YC because the public areas there are generally much less busier than over at BC. Same great location, but a much more (IMO) relaxing atmosphere overall.
All of that said, I'd pick a resort based on my park plans. If my trip were MK heavy, I'd go with Poly. If I was going to spend a significant amount of time at DHS, I'd go with BC. If I was a huge Epcot World Showcase fan, likewise, I'd go with BC. If you're planning on visiting each park just once, go with whatever resort most appeals to you. My family (including many in their late 20s early 30s) love the boardwalk area most of all for location. But many in my family LOVE DHS and Epcot's WS during the food and wine festival. A lot depends on your own preferences...but both are great resorts.