I will give you my logical reasons as to why Overeem "SHOULD" beat JDS.
Overeem usually has good defence and actually keeps his hands up to block punches. However, against BigFoot he was extremely cocky and deserved to get knocked out. He kept his hands down the whole fight and danced around like he was Anderson Silva. You just don't keep your hands down against HWs.
JDS has poor Striking Defense. Roy Nelson tagged him, Carwin tagged him and Cain kept tagging him. He is vulnerable to getting hit since he keeps his hands down low. Luckily for JDS, he has a very good chin. However, you cannot ride on the fact that you have an excellent chin for very long. Your chin will eventually crack and JDS definitely does not want to test that chin against Overeem.
I see JDS throwing strikes on the outside against OVereem, however, Overeem will block them with his hands (assuming he actually keeps his hands up in the fight) and closes the distance. Overeem will get JDS against the cage and from there, I think it is game over. Overeem has vicious knees and I think he can really hurt JDS once he closes the distance. Also, OVereem can use his kicks from the outside if JDS tries to keep his distance. We all saw what Cain did to JDS legs in the first fight in which the whole fight happened on the outside.
The good thing about JDS vs Overeem is that it is proven from JDS's fights that if the fight goes more than 2 rounds, his power goes and of course, the threat of a knock out goes. He has slowed down a lot against every fighter once the fight hits the 3rd round. Of Course, Overeem also has a terrible gas tank. So I think we could possibly see 2 gassed bums fighting in the 3rd round.
Nonetheless, Overeem has the strength, wrestling ad BJJ advantage so he can always use that if he needs to.
If Overeem fights like he normally has over the past few years, I don't see JDS beating him. We may see JDS knocked out for the first time in his career if he actually gets cocky against Overeem.