I'm looking for a tool to remove gun barrel and other cylindrical-object join seams, and can't decide whether to get a flex-i-file set, or a seam scraper from Micro-Mark.

My two cents - get a big sheet of sand paper and cut it to various sizes. Then use the papers ability to mold and fold.
Go to a hardware store and go to the painting section (at least that's where it is kept here) and get the high # grain. The paper usually is $2-$5 usd for a few 10"x10" sheets. It ends up being cheaper and more useful. You can custom cut the paper to fit each need.

If you are set on getting one of the two tools I would get the flex-i-file for the job you're mentioning. The seam scraper can be duplicated by the back of a hobby knife.

The flex-I-file is nice. Like Slodder said the back of a blade works as a seem scraper too. you could use a Curved blda as a scraper too