This really depends on the resources, technology, and expertise available to each effort. Generally speaking, establishing colonies in space and other planets would be more difficult and require more resources, but would also open up more possibilities and have more long-term potential. Establishing underwater habitats and colonizing the oceans of Earth would be easier and require fewer resources, but could provide more immediate rewards in terms of access to resources. Ultimately, it comes down to the capabilities and goals of the people attempting to establish the colonies.