Hello sfbank! Poly and BWI are among our family's favorite resorts. I love both for different reasons, but they are very different. I love Poly for the theme, the tropical feel and the fun, relaxed atmosphere. I've always preferred the Poly pool over the BWI pool - just feels more relaxing and really enjoy the view of the lagoon. I also love the proximity of the Poly rooms to the pool and the beach - particularly when staying CL in Hawaii. So convenient. I'm a fan of the dining options at Poly, and it's nice to have the monorail access direct to MK and then direct to Epcot from TTC. So excited about Trader Sam's - love the DL version and bet this will be even better with the outside seating. The view from the CL lounge is really wonderful - nice to watch the fireworks from the two-level lounge. And in the lounge, there is plenty of room to spread out. I particularly enjoy the upper level of the lounge - entirely uncrowded most of the time.
On the other hand, I do miss the easy access to Epcot when staying in the MK area. Ideally, we'd have dinner in or around Epcot every night and as you know, BWI can't be beat for that. But Poly is second best, since we can take the monorail direct to TTC and walk from there. One of the biggest differences is the lounge atmosphere. Very casual, laid back atmosphere in the Poly lounge. More guests using evening appetizers as dinner or ordering in/eating dinner in the lounge at Poly. More guests getting drinks for the pool, and occasionally some with a few too many drinks for the pool.

The CMs are nice at Poly, but we've not encountered anything at Poly that's similar to the Jackie/Pete/Scott/Michael/Anita combo at BWI. And in our experience, the BWI lounge has always been more quiet and super well-maintained - probably because it's smaller with no separate kids' area so it's easier to monitor.
So as I mentioned, I like both for different reasons. I really love a Poly CL/BWI split stay!
P.S. As for tips, everyone focuses on the monorail, since it's a benefit of staying at Poly. But I really love the boat to/fro MK - so relaxing.