Thats tough because in those years you mentioned (and keep in mind there was no DTS in 99, just Deville) the cars are actually very similar in performance and handling characteristics (not to mention they are just a re-hash of the same platform anyway).
In 2001 the STS got a bit of the edge in the technology department with Xenon lights, bigger wheels, and some interior enhancements.
It will come down to which style fits you best, Its basically like the difference between a Rolex Submariner and a Rolex Datejust watch. Same internal works pretty much, but different exterior character. Sporty vs. more elegant. I will say that the DTS is wearing a little better with age than the STS, even though I always preferred the STS myself. This is partially because the new 06 DTS is still based upon the same car.
In 2001 the STS got a bit of the edge in the technology department with Xenon lights, bigger wheels, and some interior enhancements.
It will come down to which style fits you best, Its basically like the difference between a Rolex Submariner and a Rolex Datejust watch. Same internal works pretty much, but different exterior character. Sporty vs. more elegant. I will say that the DTS is wearing a little better with age than the STS, even though I always preferred the STS myself. This is partially because the new 06 DTS is still based upon the same car.