I don't have any experience with the original insert, but we are still using the heart 2 heart for our 2 month old. He is 13 pounds now and almost ready to go without, but it has been a good investment. We use the Ergo whenever we are carrying him on an outdoor, longer trip, and we use the Moby and the Kangaroo pouches for indoor and shorter outdoor trips (DW seems to have bought every system out there!)
The insert seems to keep him very warm, which was a big plus in the winter, but would be a big disadvantage in the warm Disney months. I would use a Moby or similar for an infant at Disney who is too small for the Ergo style. The Kangaroo pouches just don't seem comfortable enough for us for long wear.