The pool views face in toward the center of the resort, overlooking the pool areas - I believe the pool views start on the ground floor and go up to like the 6th or so floor (there is a pinned thread at the top that is very detailed and has the exact breakdown). The Island garden views face the outside of the hotel and depending on where you are, look out over the spa, the island, the lawn where the luau is held. We have stayed in an Island Garden twice and been happy with the views both times - first time we were right over the spa and could even see the ocean a tiny bit if we looked that direction. Second time we actually had a great view of the ocean and the lawn - we could watch the outdoor movie and the luau from our balcony (though we couldn't hear it perfectly).
ETA: I realized I didn't really answer the question. I'm not sure what makes it a better view. I thought our last Island Garden view was ideal. It was practically an ocean view room. Probably just preference - I have heard ground floor pool view is nice as you just literally walk out your back door and are right there at the pools. Also, the activity of the pool area can be fun to watch and it is beautiful light up at night. So, my opinion is it isn't better - just what you prefer.