Aquarium or pond?

I want to get a couple goldfish, so I'm doing a lot of research before I jump in. I decided to upgrade my aquarium to a 75 or 100 gallon and was thinking about getting 2 comets. I know they can get quite large and I've read that a lot of people end up putting them in outdoor ponds. Is this a need or a preference?Obviously more space is always better, but can 2 comets live happily in 100g? Or should I opt for a smaller breed? I really want a larger variety, but I want them to thrive, not just survive.