The 2.0 Duron will mop up the Thorton at 1.67, to be sure.
But having a Thorton that wont OC probably wont happen. Thorton's, being nothing but a Barton with half the cache, overclock very similar to the Bartons. 2.012 Ghz seems to be an easy overclock for the 2000+ Thortons, judging by most forums.
Locked 1.6 Durons and Thortons ARE hitting shelves, so its possible that you could get one. I didnt, and have been lucky enough to hit 2.1 (10.5x200) at 1.65v no problem with my 1.6 Duron. If you do get a locked one, you will have to work the FSB a little.
Overall, both chips are similar performance wise. Its rather easy to unlock the extra cache on the Applebred, but its also easy to unlock the full 512k of cache on the Thorton.
If you really have no way to overclock the Thorton, the Duron at 2.0 is a better call... especially once you open the rest of the L2.