Depends on the year and models.
1971 and 1972 are most valuable as originals, without question.
1973 and later XS's up to when the "Specials" came out are a toss-up IMHO. To ME, they're also worth more in original form, but to others, who knows?
Specials are so plentiful that anything other that a truly pristine, low-mileage original can be modified in any of the usual genres and be worth at least as much as a clean original, and often more.
These are my opinions only, and are not based on actual experience or research.
In other words, I could easily be full of shit.