I've stayed in both. We're in Long Point right now and like it for the peace, price, relaxed atmosphere, and convenience to friends in the area. With tax, we're paying $31 a night for two-night stay, about the same price for both places.
Overall, S. Inlet is nicer, but LP is much easier to get into and fine for a short stay. As of last year, they have online reservations and confirm site number when you book. We had to change our arrival date for a medical reason, and Long Point did not charge any fee. Grass sites, dog friendly, one nicely updated bathroom plus older one, lots of puddles when it rains heavily like it did yesterday.
Like Sebastian Inlet, some sites at LP are at the edge of water with wonderful views, while some back into marshy mangrove area. Both places can be buggy. Both are popular with fishermen/women.