I would pick the Poly for the reason that you haven't stayed there. Poly was our family's favorite resort (pre-renovations) and we own at BLT, so we like both.
We love the new salt water pool and eating options at Poly better, we like the jogging trails and fitness centre (but it's at GF so a bit of a walk but the stroll between the two resorts is lovely). We like the quick access to EPCOT (we usually use the TTC rather than Poly's pickup location).
We like the fireworks better at BLT as the viewing area at Poly is now small and often crowded. It is sometimes hard to hear over the less interested people who prefer to talk during it. For us nothing beats seeing it from in front of the castle or your own private balcony at BLT. When I was in a poly lake view room last August, the music channel for Wishes was not available so watching from the room was a bit pointless. Not sure if that has changed.
I won't go on about BLT since you've been there. When it's our whole family going, we always choose BLT. In smaller groups, our resort choice depends on personal taste and where we want to spend our time.
Also, be prepared for a "bungalow view" if you choose the Poly as 1/3 of the LVs have no view of the lake. I was given a ground floor handicap room last trip and while the porch was a peaceful spot to have breakfast each morning, the only lake I saw was the boat docking at the ttc to the far right.