As a general rule of thumb, older+smaller=in demand. Another generality is that the more panels on the end cap, the more people get excited about it. What markdoane says is essentially true, but I think it's more because of supply than anything else... those were the peak production years for Airstream, so there's a lot of inventory out there.
Personally, I'm not looking for collectible, I'm looking for a trailer that's pleasant to travel with and camp in. For that, a good '70s trailer is pretty nice. Lots of large windows, a variety of floor plans and sizes, relatively simple systems. Of course, they're 35-45 years old, so most of them as-found are not "a good '70s trailer" and will require some work to get into good traveling condition. That said, unless you spend a lot of money on a trailer someone else has restored/renovated, you're likely to find most of the same problems in a '60s trailer as well.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill