moreton island and north stradbroke island

Yeah, no. Both are great.

It's best if you can get there during the week days and outside public / school holidays because it does get a bit crowded.

Straddie hands down, much more to do without a 4WD and it is so relaxing. Stay at Point Lookout and you can't go wrong. If the Gorge geometry dash lite is shitty swimming conditions due to wind direction then Cylinder Beach is usually good, and vice versa. A daily headland walk and pitstop at Brown Lake on your way back to the barge are musts!

Moreton is more like Fraser - all sand driving with a couple of hamlets.
Straddy is more like the mainland - mainly road driving with a few small towns to visit and some beach access. Has coffee and pubs and restaurants etc etc.
Both are great in their own way.