In brief
Decide how often you will actually move, and how many people travel, then pick the smallest category that satisfies both. A couple who travel at the weekend and leave the caravan 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 this 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 caravan 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 unlawful in Switzerland. How self-sufficient do you need to be? If you want to camp without a mains supply, the power and water plan matters more than the interior finish.
Streamlined aluminium versus boxy: a real trade-off
A streamlined aluminium caravan tows better and holds its 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 regrets
Check the roof first, the slide-out seals second, and the bathroom and kitchen sealants third, because these are where age shows first. Have a wet-area inspection done before you sign anything.
The ownership costs people forget
Insurance, campsite season fees 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 caravan 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.