We did a split stay last 4th of July: Contemporary CL for the first 2 nights and then onto YC CL for the remaining 4 nights. The monorail hotel stay was done so we could readily access MK and AK; the Epcot area hotel was to be able to visit HS and Epcot with ease.
I enjoyed the Contemporary much much more than I thought I would and would gladly return. Still, I preferred YC, not only because of it's proximity to Epcot and HS which are our favorite parks but because the staff and setting of the club level was more comfortable and better set up to my way of thinking.
I'm an early riser and was frequently in the Atrium at 6:05 AM. Set up was generally not completed until around 6:20 and oh the horrors!

D), once I had to turn on the lights myself and there was no one around to tell me how to operate the Nespro (sp?) machine or written instructions on how to use it. The lounge needs another fridge (think there was one for sodas). The hors-d'oeuvres hour ends at 10:00 PM I believe and if you arrived at 9:30 PM it was warm beer and wine in melted ice tubs, many tables that needed bussing and little of nothing left to nosh on. If the staff knew you from past visits they were quite chatty but less so with "newbies".
The views of MK from the lounge were fantastic and we opted to watch fireworks from there rather than enter MK to see them. Lovely on point furnishings and decor yet I wish the space was enclosed rather than being open to the elevator lobby. A glass wall could have done this nicely while still keeping on point with the mid-century theme.
When we entered the YC lounge it was a new and better world. Staff was knowledgeable, morning setup completed on time and you didn't feel you were imposing if you asked a CM a question; just much more approachable staff. Cocktail hour and breakfast offerings were better and more varied. Enough space and refrigerators for cold beverages no matter what time of day; they even had a glass-fronted freezer stocked w/ ice cream which the children appreciated a lot.
Constant refilling of food throughout service hours and cleaning of tables after prior guests left.
The view from the YC lounge was no where near as outstanding as that of the Atrium Club as it overlooks rooftops and a bit of the pool but the overall space did a nice job of making you forget that.
All and all, YC sold me best on the additional CL cost and it inspired me to give BWI a try this coming Thanksgiving. I've read a bit on the Broadwalk Inn's club level and it sounds like it will live up to our YC experience.
HTH!