a lot of the trade space will be driven by where the boat will be used (state) and their requirements as well as your intended use. Recently, the wonderful federal government started an annual fee for the documentation renewal so that will need to be compared with the state registration (usually, you'll do one or the other but each state has their own 'rule'). The biggest driver FOR documentation is commercial use ("for hire" services on the water) - it is required for many of them. Another driver will be any loan on the boat - many lenders prefer documentation as it is recognized across the country (lowers their risk)....a final use where it would help is if you are plan to visit a foreign port....