I've had the sheath on the fiberglass beaded tc's discolor and melt in an iroast. Once they get crudded up they aren't as accurate, and cleaning them with any vigor risks the weld. The stainless steel clad probe does not lag any more than a beaded in this application, the larger diameter improving the stability of the readings and much easier to get it where it needs to be.
I like to try to find things rated for food contact when possible, perhaps unnecessarily.
On a side note, the DuPont website has this note:
DuPont non-stick cookware may produce fumes at significantly higher temperatures, greater than 500°F (260°C).
http://www2.dupont.com/Teflon/en_US/keyword/fumes.htmlThe are talking about thin layers of teflon coating, (who knows what the properties of the sheathing are, it is not pure teflon) but this seems awfully close to roaster temps.