Rose gambled on baseball while he was actively participating. For the most part that gives the game a bad image, though there is the possibility he threw some games to win. He says otherwise, but he's denied everything so inconsistently I wouldn't believe him if he said the sky was blue.
A-Rod was juicing and likely has been doing so his whole career. That also gives the game a bad image. What it also does, is inflate player statistics and effects salaries of more than those who juice. So that actively alters the results of the franchise, budgets and player statistics for years. That's why I think it's worse.
Honestly, I think Selig's just as responsible. He's turned a blind eye to this for the last twenty years and is only now coming down hard on this when his time as commissioner is coming to an end and he's playing damage control for his legacy.