The `type` of the planet has no real effect on what happens there beyond disallowing certain types of infrastructure. Planets do not have to be self-sustaining in this game, meaning you can have one planet do all the food production for your empire and the others focus on commercial economies, or you can even do no food production and buy it all from the market.
Most people plunder Gas planets due to their lack of much building space. Barrens have the potential to have a lot of space; most people, when looking for the planets they will use to ultimately build their empire, look for barrens with 700 or more land. The more picky players will only accept 800, while some elitists will refuse planets with less than 900 or 1000 land in their clusters.
Essentially, you want your planets to have as much land per planet as possible; U class planets and large Barrens help achieve this goal in the most realistic manner.