We own at the Islander and at the Sands. We love them both. They are both every different though, and it almost feels like a completly different vacation going to Islander versus the Sands.
So many people have mentioned the differences in this post, but I will summarize the differences that matter to us!
Pros:
Islander
1. One of the Tri-Royals (love to be able to just walk over to the Mayan and Carribean so easily. You can pick a different pool area every day which makes the vacation seem more flexible and interesting).
2. More Homey feel - as others have mentioned, it does have more of a homey feel to it. I even think the staff at the Isalnder is nicer.
3. Looks more tropical with the curvy shaped pool, landscaping, etc.
4. Better Pool bar food menu
5. Closer to Captain's Cove
Sands
1. Close to main road, easy access to bus stop (no huge hill like the Islander has)
2. Close (walking distance) to the malls, restaurants, etc.
3. Beach access at Phase I is same level as pool - a HUGE benefit in our eyes.
4. While the Phase I pool area is huge and somewhat sterile looking, you can always walk over to phase II and get the tri-Royals curvy pool area feeling.
5. Grocery store is much nicer and has a lot more items.
6. There is a sports bar at the Sands.
7. Coffee shop at the Sands
Cons:
Islander
1. Beach access is 4 levels lower than pool - you cannot see the pool if you are on the beach
2. Islander beach is a lot more rocky and narrow than the Sands (this could change in time, but for now, it is what it is)
3. Islander is situated on a huge hill that is a challenge to walk up in the humidity and heat. (However, there is a free shuttle you can catch every 15 minutes down at the bottom of the hill at Captain's Cove, so this is kind of a wash).
4. Islander is futher away from the "action" of Cancun. Not really walkng distance to anything.
5. Laundry area at the Islander is very small compared to the Sands.
Sands:
1. Phase I has a very non-private, sterile feel to it.
2. Pool bar menus are not as good as Islander menus.
3. Staff is not as nice as tri-Royals. (They are ok, don't get me wrong, but the front desk at the Sands has always been an area of concern for us...they can be the rudist people you have ever met).
I think you mentioned week 16 as well. We own weeks 16 and 17 and go almost every year during those times. In fact, we already have our plane tickets for next year!

We love going during those weeks. It is usually warm, but not oo hot, and very little rain. Good week to pick!
Michelle