If Phelps' PR people were able to divert the black eye of a DUI, I think they will make this disappear from the public mindset even faster. I consider it a non-issue. He fessed up, apologized, acted mature, and he has the support of his sport's governing body. He was given a slap on the wrist, and once he's in the pool again, this will disappear.
A-Rod, on the other hand, is a super-star on a team laden with super-stars, and he was brought in as "the best player in baseball" for the year he was using them. Ironically, though A-Rod broke no laws and Phelps did, A-Rod will come under much more scrutiny. Baseball is a bigger sport here in the US than swimming is. Phelps is in our face once every 4 years when he shows up to bring us gold. A-Rod is in our face every day for 6-7 months of the year, every year.
No comparison... I won't say who screwed up more, but I can tell you who will be screwed over more.