Constructionwise, compared to a strat:
-Thinner body (initially without contours)
-less routing
-one fewer pickups
-one fewer knobs
-less wood in the neck
...But then again, so did the Jag (aside from the body issue) and it was top of the line. And by the virtue of its bridge, a Duo Sonic seems much more low end, but then the Telecaster was similar in that regard... (Perhaps the MM losing the pickup and both switches as well is a bit more low end...)
Additionally the Mustang had:
-Chrome plates
-More switches
-(Arguably) more complex bridge/trem arrangement
...Again, as did the Jag. In the end, it's hard for me to see the exact aspects that made up the Fender heirarchy in those days...
In these days I think of it more in the way that Porsche had its lineup horizontal in the 80s: The 911, 944, and 928 all offering about the same kind of performance in fairly different packaging. But I still can't figure out why Fender initially called Mustangs "student" guitars, or Jags "premium" for that matter. By price, a Tele had to be far cheaper to build than a Mustang...
My biased opinion tells me that Jags are at the top because they ROCK more
