I have not stayed at either resort...
However...on my last trip (a month ago)...DW and I decided to spend a few hours resort hopping via the Monorail one evening. This allowed us to spend about 30-45 minutes at each of the 3 Monorail resorts to take in the "Disney". Based on that...
Poly: From the beach you pretty much get a direct shot at MK. You can see Cinderella's Castle and Space Mountain...but they are off in the distance. The grounds at the Poly are perfect if you want to "get away" as you walk back and forth between the bus stop/Monorail/your room.
Grand Floridian: You are closer to MK, but you do not get a direct shot at it. So, if you are wanting to watch Wishes...it would look cooler from the Poly. A benefit to the location though is that you are the last stop on the Monorail Resort Line. So...it's a quick trip to MK from there without having to stop at the other places along the way. Of course...this means when you go back to your resort from MK you will have to make a few stops along the way...unless you take the boat I think.
Both places are amazing and I would give anything to take DW to either one for our first anniversary (only 11 months away

). They both have Disney magic without being "too Disney"...if that makes sense. In other words...you can completely escape the crazy side of Disney at these two resorts...and still be close to MK.