EIther should last a long time. Wax, probably longer. If your toilet is not shimmed properly so it doesn't rock, a waxless migth keep it from leaking. A waxless is more forgiving on the depth of the flange (i.e., one where the floor has been built up around it) since it has a built-in horn. The waxless rely on rubber which is pretty reliable (often neoprene or other synthetic), could break down in certain circumstances (but it should be a long time). Wax works, is cheaper, but can slip and is messy. If you have reason to want to remove and replace the toilet, the waxless is quicker, since you don't have to scrape off the old one prior to installation of a new one. This is particularly handy during remodeling when you might need to set and remove the toilet a few times as you progress in the remodeling.