I prefer expanding into islands that have already high resources, something around lv16 luxury and wood. I don't mind if they're not beside my other towns, I can deal with up to 3-4 hour transport time.
Moreover, I prefer having my towns scattered so that when I make enemies, they can't find/attack all of my colonies and attack. :3 It also give me more trading options and more *cough* pillaging targets.
You can use this tool to search for islands: Ikariam-World - Search Player