The boat from MK to GF is the fastest. The monorail from MK to GF may or may not be faster than the boat, but for the kids it's pretty darn cool.
But, many (adults and kids a like) think the monorail is so fun that it's as good as any ride in MK. So, you may WANT the longer ride from MK to GF, just so you are on the monorail longer. It will giveyou a bit of a tour along the way, the "announcer" points out interesting sites and facts about the CR, the TTC, the Poly and the GF.