looks like you've done your homework and far, far in advance, good for you!
since you are so far out i'd watch for some great packages and improved rates as your dates get closer. rates are very dynamic!
i walked over to the andaz last week to check things out and it looked nice just more people and long lines at the beach hut (not as populated as grand wailea tho). like you, i'd stay at the andaz with a price differential as you've quoted. i would just keep in mind these 2 properties are nothing alike.
the fs maui, although not perfect like hulalalai
, has fine-tuned the minutia and personalized service which makes for an unforgettable, memorable trip.
* just a note on montage. i checked that place out, too, and found it to be gorgeous, gorgeous! i didn't book due to the crazy-high rates on my particular dates, just couldn't justify the cost. i'd be more inclined to go the montage route if traveling with my family in its entirety.