The short answer
Decide how often you will actually move, and how many people travel, then pick the smallest category that satisfies both. A couple who travel weekends and leave the rig on a pitch do well in a compact towable. A family that moves every day needs a self-propelled rig. The most expensive mistake is buying a vehicle and then discovering that the route you want is impossible with it.
Four questions, in order
How many people, including children, travel at once? Sleeping capacity is the first hard constraint and it decides the class. How often do you move? If it is every day, a motorhome; if it is weekly, a towable you leave behind is more comfortable and usually cheaper. Where do you go? Alpine passes, old towns, islands and ferries punish height and length, and a fifth-wheel that is perfect in Texas is illegal in Switzerland. How self-sufficient do you need to be? If you want to camp without hookups, the power and water plan matters more than the interior finish.
Airstream style versus boxy: a real trade-off
An aluminium streamline trailer tows better and holds value well, but its rounded shell wastes floor space and its layout rarely varies between models. A boxy caravan offers more usable area per metre of length, better insulation options and cheaper repairs, at the cost of more wind drag and more sway. Neither is better; they suit different kinds of traveller and different kinds of route.
Buying used without regret
Check the roof first, the slide-out seals second, and the bathroom and kitchen sealants third, because these are where age shows first. Ask for the service history and any modification receipts. Test every system with a mains hookup and under load, not just with the engine off. And budget for the first service: tyres, bearings and batteries are the three items most likely to need immediate attention on a used rig.
Ownership costs people forget
Insurance, a campsite season or per-night fees, gas, storage if you do not keep it at home, a service contract, and a sinking fund for tyres and batteries. In many countries a towable is cheaper to insure and to store than a motorhome, and the motorhome's advantage is that it does exactly what it says on the tin when you want to move.