Enclosed or we wouldn't even purchase it. Had several of both enclosed and open over the last 52 years and the difference in winter and summer comfort, floor temps, resale value, rusted frames and rotted floors from soaking and drying constantly over time is huge and usually very costly to repair or scrap it.
The open underbelly trailers always had very cold floors in 40 degree or under outside temps and thus cold air above the floor too. To counteract this, you needed to raise the thermostat temp setting which means your head is in the hot stuffy air while your feet are very cold. Warm air rises and cold air sinks! Not pleasant to have this uncontrolled at all and in this modern age, using barbaric yesteryear thinking! Go ask any RV dealer why the higher quality RV,s all come with enclosed and insulated underbellies as standard? You'll have your answer real quick! BTW, yes, enclosed under bellies on RV's also helps with summer heat and not having hot floors. Insulation and a dead air space helps minimize discomfort with/in both hot and cold climates just like in a home!