Yeah the 500 would be more space for sure.

Also look into CPR's hang-on fuges.

Not being cute but might want to think about the purpose of what you want to do.

A 'fuge' on it's own is a place to avoid predation. Are you looking for a place for pods, want to grow chaeto etc.. The size, shape and media within it (if even needed eg. why for chaeto) will differ depending on the application.