CostCo's a great deal. I don't do it because I have an AP and thus, have already paid park entry.
CostCo, I understand, includes the resort fee, but, I'd check on that.
There is a parking fee if you have a car/rental that everyone has to pay for.
Swolphin's infrastructure is not fully connected to Disney. Once you book, it will take about two days for your reservation to hit the Disney web site (My Disney Experience) at which point you can book FastPasses at 60 days. If that doesn't happen automatically, call guest services.
With Swolphin, there's no MagicBand, charging back to your room, or airport transportation. Otherwise, it's just like being in the Disney bubble with Disney transportation to the parks and Disney Springs.