Certainly steroids can be used safely, and to be clear, I think that the healthy lifestles lived by users of steroids should more than make up for any aggragate negative impact steroid use may have. I fully support the use of anabolic steroids, they are a-okay in my book.

However to say that pot can be dangerous because it's users often adopt fatass lifestyles seems a bit disingenious. You could similarly start attributing sports injuries to anabolic steroids (clearly sporting injuries would never catch up with fatass injuries, but anyway..), though that would be similarly disingenious. I think the only reasonable way to compare the drugs is to compare the harm that the substances themselves are capable of inflicting when taken in dosages that are logistically feasible.

In your fictional situation I think I would choose the unstated option c) clean up my diet, smoke pot occasionally, and go to the gym almost every day. Hmm.. come to think of it, that is already what I chose...