The external will be the easier one to do maintenance on. However, if you go that route you will need a pre-filter on the pump. Sometimes those can be a pain to deal with because they clog so much faster than putting a filter before the pump. Have you thought about a skimmer? Using a surface skimmer will keep the debris from sinking and will make the filter work less. Check out koiphen.com and go to the construction forum. There are lots of diy projects you can get inspiration from.