The triple 6, 16660 originally came with a matte dial and no white gold hour marker surrounds. Then later essentially the same style dial as the 16600 (all in tritium) - the only real difference being an update of the movements. Later the bracelets would update on the 16600 too.
The 16660 is more desirable, especially if it has the early dial, and was only in short production before the 16600 was launched.
Personally, I'd buy on condition and price between them, and would probably only buy the 16660 if it was an earlier dial version.