If you will have a car with you, you may need to consider parking charges along with the rate. Last time we stayed at SF, they charge the same rate as the deluxe do for parking (which is more than CBBR and Aventura charge for parking).
One thing about the water taxi is they do not run when there is lightning in the area (all the hotels have shuttle service as well), but this should not be much of an issue in DEC. we stayed at SF for HHN and ended up walking to the park each night as the storms rolled in as we were ready to go to the park each night.
Me, I love the walk (my wife and daughter do not). You end up in City Walk near Margaritaville (so closer to IoA than were the water taxi drops off).
My guess is that Aventura will have some better views. I also assume there will be times waiting on elevators (as I believe there are more floors). So SF is a little more spread out while Aventura is more stacked, both hotels are in the same area, but I suspect that Aventura is meant to be more ‘no frills’ than SF is.