Boardwalk, unless, as someone pointed out, you want the best pool bar. The pool bar is better at Poly. The pool experience, in my opinion, especially for adults, is BW. The Poly pool area is very, very crowded. I also thought the pool itself felt smaller than the one at BW, but that could have been due to the number of people in the pool. I found the quiet pools at BW actually quiet. No one was swimming in the one on the Inn side the day we tried it out last July (around 4 PM, no thunderstorms). The Oasis pool, on the other hand, still gets quite busy; going back to my first point, there is a pool bar at the Oasis pool; to the best of my knowledge, neither quiet pool at BW has a pool bar (only the Luna Park pool does). So if the pool bar (or the water slide...adults can like water slides too!) is important, I'd go with the Poly. If you want a quieter, more relaxed pool and/or poolside experience, I'd go with BW.
Personally, I love BW for so many reasons, one of which is that I prefer the pool experience there . My DD loves the pool at the Poly (namely for the water slide), although she also likes the quiet pools because they aren't so busy. She didn't want to spend any time at the Oasis pool because she found it almost as busy as the Lava pool; she loved the quiet pool at the BW and wanted to know why the Poly didn't have a pool like that.