We've been asked this question for many years and it will continue to be a good one, especially given the amount of "false info" out on the internet these days, with all sorts of "reviews" being written and taken as "truth".
Both of these sides of Maui have been developed for "tourism" by virtue of their geographical location, though the westside was developed first, mostly in the 70's and 80's because of the prior existence of Lahaina town, with the southshore being a little later to develop in the 80's and 90's.
Here's our opinion, along with some facts, about the southshore of Maui vs. the westside, based again on 25+ years of not only living here but guest feedback during all that time:
1) The southshore typically has warmer and drier weather thru the critical "winter/spring" months of December thru April. While both are "leeward" shores, the westside gets about the same rainfall and wind conditions as the northshore. The southshore tends to be much less rainy and even less windy the further south you go.
2) The beaches, all of which are open to the public, including the ones
in Wailea where the hotels are located, are superior not only in number but they could be called more "user-friendly" in that they are less prone to get surf most of the year and provide better swimming, walking, and snorkeling given their geographical location on the island.
3) It's more centrally located so that you can use it as a good "home base" from which to enjoy the whole island, which is really what a vacation on Maui is all about, without being somewhat "isolated" as you would be staying on the westside. It's just a 15- 30 minute drive to the northshore and 30 - 45 minutes to the westside, depending on where you're staying on the southshore.
4) There's less "density" of development on the southshore in terms of
condos, hotels, and commercialized areas. Of course, no one is attracted to the "suburban sprawl" of the southshore, that's not what you come to Maui for, but it is nice to have modern conveniences of shops and restaurants close by without having to "look" at them or be surrounded by them during your stay, which is why we only recommend properties in the southern part of the southshore usually, in or near Wailea, not in mid-town Kihei, so that the busiest parts are "out of sight/out of mind".
5) The sunsets are better. (OK, that's not true, sunsets here are
wonderful just about everywhere.)