Luthor might kill you if you fail or are of more use to him dead, but the Joker might kill you for a laugh.
Also, if you're in Joker's gang, you'd go up against Batman, while on Luthor's team you'd be more likely to go up against Superman. Chances of victory against Superman are vastly worse, even as bad as they are against Batman, but Batman might break some bones. Superman takes greater care not to seriously injur his human opponents. Finally, the Joker doesn't really care about money, so you're probably not gonna get rich around him. Luthor might not send you to rob banks or anything, either, but he's rich enough that you'd probably be paid fairly well as long as the gig lasts.