It all depends on your needs. For most people, regular P-metric and LT-metric sizes will be more than capable enough on most off-road terrains. However, if you are into visiting some very remote places and driving over very demanding off-road terrains, it might be a good idea to invest in flotation sizes. These tires allow you to drive at very low pressures, which can significantly improve the traction for rock crawling and when driving over sand dunes.