I have both but haven't played Monte Vista much at all yet. I may be in the minority here, but I really like Sunlit Tides (except that it takes forever to get around the island--but I just added llama stations all over the place, so that should take care of that problem!).
I have Seasons, but am only playing summer in Sunlit Tides. I just love the tropical feel of the place, and it is big, and there is plenty of room to explore. And the sunrises/sunsets are gorgeous there. I like the prebuilt houses quite a bit too.
I don't tend to play with townies at all, so can't comment on which are better.
Monte Vista is pretty, too, but I like the spaciousness of Sunlit Tides (even if it causes transportation problems!), and Monte Vista just isn't personally as appealing to me. So I think it will depend on lot on what kind of landscapes you find aesthetically appealing, too!