Having done both (on and off) I must say it totally depends on the off site hotel to me. Only one hotel (out of 6) that we have stayed at offsite was as magical to me as staying at the Grand Californian. The offsite hotel we loved was Candy Cane Inn. It just had its own magical feel to it. The very best part of staying onsite (at GCH) was walking right to your room after a long day in the parks. Have fun deciding. Maybe you need to do 2 trip to try out both options?!?
