Staying onsite and getting the resort pass usually works out cheaper, and it is unlimited, so definetly well worth it! It's also valid from the day you check in till the day you check out (we've done a one night stay before and used it both days!). I'm assuming that it'll be the same deal as the International Gateway and boat between DHS and Epcot at WDW, and you probably will need a park hopper to do both in one day.
Also: keep in mind that, whilst Dragon Challenge and Flight of the Hippogriff both have Express Pass, Forbidden Journey DOES NOT. Waits can get ridiculous, but by staying onsite, you can get early entry, which definetly helps!