Wall air conditioners versus mini ductless air conditioners?

So which is better the wall/window air conditioners or the mini duct split system air conditioners? Reason asking is about to get a mini ductless system for a trailer I have. The window/wall air conditioner here seems to last a year to 3 at max and gotta replace them cause its more expensive to have them fixed and aren't easy cleaned especially on 2 ton units.