If you will be using the monorail at other times, then I highly suggest the boat from Poly to MK. It's just interesting and different. And on the way to Poly it will make a quick stop at Grand Floridian, giving you a unique and up-close vantage point of that resort.
And when you get off the boat at Poly, you are in the "backyard" of the resort, along the lagoon, so again you get an interesting view of the resort. Arrival by monorail is very convenient, as you are on the second floor inside the Great Ceremonial House, just steps away from the two table-service restaurants ... but the arrival by boat is more scenic.