All I can say is, we visited the Airstream factory and the factory that built our last trailer, and the difference was quite striking. We were very impressed with how the Airstreams are built. As DownTheAvenue pointed out, they tend to last many, many years (as long as they are maintained properly, of course). And it's also true that they lack headroom and space, and probably tend to be a bit heavier, compared to most TTs of similar sizes.
Not sure you'd be happy towing a trailer with the 4Runner, judging from our experience towing with a midsize SUV (Ford Explorers, in our case). One thing you'd get with the Airstream or a Lance would be a bit of a lower profile, which would help. It's still going to be a tough haul.