Assuming your touring pretty much on asphalt and you'll accept the occassional flat IMHO the Supreme is the best currently produced tire out there. If you want "zero" flats and better durability and are willing to suffer tires that are twice as heavy as the Supremes you'll want the Marathon Plus - which I would say is the best commuting tire available.

However, if you can find Marathon XR's, they will be more durable, puncture resistant, and cheaper than the Supremes, but they'll be heavier and have less grip on wet surfaces. And I say this with 20k actual touring miles using combinations of the XR and Supremes.

As to your "is it worth it" query, it depends on you. If you expect to see lots of bike shops in the course of your tour and you're not picky about what you use, then it could be argued that a succession of cheaper ($30 retail in a bike shop?) tires is better than one best quality tire that may or may not outlast them. Personally, I'd invest in the best tires to minimize potential downtime (such as searching for or hitching a ride to find bike shops) while on tour.