Miyata was unique in that they rolled their own tubing, as well as selling said tubing to companies like Univega. Hard to go wrong with any of their touring line. I wouldn't buy any bike for my personal use based on rarity - I'd buy the one that fit me best. Fit is your most important consideration, everything else follows.

If you're looking at a true touring bike, weight of the tubing - and whether it's single/double/triple butted etc. - is not as much a consideration as you might think. Durability trumps weight, or the lack thereof, unless you're not using it for touring but instead as a commuter etc. (I commuted on two C-dale touring bikes over 20+ years.)

Having plenty of braze-ons, three water bottle mounts, long chainstays, cantis, plenty of clearance for wide tires & fenders - that's where the interest lies for a true touring bike.