JDS would be favorite over Overeem, steroids or not
If and when the fight between JDS and Overeem happens, I see JDS being clearly the favorite.
Overeem may have been the K-1 grand prix winner, however they compete in MMA. JDS is training wrestling and BJJ with the Nogueiras and other Brazilians, and I believe he will have an advantage over Overeem in that despite the size and strenght advantage
In terms of striking, Overeem is not the best in K-1 even if he won the grand prix. JDS is considered the best striker in UFC by boxers like Klitschko and David Haye.
Going by the logic that JDS only uses his hands and kickboxing is better then boxing, then a top k-1 fighter like Mirko Cro Cop should have beat JDS by striking, but he got stopped.
Also note that Overeem has a suspect chin, and was rocked fighting in K-1 by people no bigger then JDS.
Just because JDS never competed in K-1, doesn't mean he's not a top striker that can't beat Overeem in MMA
Following: Cain , JDS, Werdum, Shogun, Machida, Weidman, Pettis, Aldo, Melendez