For our trip in December we had both a backpack carrier and a stroller for our 15 mo. old son. We used both. My son prefers the backpack, but I brought the stroller, hoping he would fall asleep in the parks. (That NEVER happened. He didn't nap ONCE!) He was often in the stroller during lunch and snack times, or while waiting for his Dad and brother (3 1/2) while they were enjoying the "bigger" rides. There were a few other times he enjoyed sitting back in the stroller while we explored the parks, but for the most part preferred the carrier. Fortunately, the stroller didn't go to waste as my 3 1/2 year old often got tired and needed a "ride".
When we weren't using the backpack, I stored it in the basket under the stroller. It was very easy and not as much of a hassle as you might think. (I didn't bring the Baby Bjorn, as my husband prefers the backpack carrier, and DS has pretty much outgrown it.) DS loves, loves riding on his Dad's back -- it's a big thrill for him -- and he gets bored in the stroller after a while. A few times we left the stroller at the resort and just used the carrier. It was much easier to get around (especially getting on and off the buses) without having to lug the stroller.