They're both long shots but I think being a rapper would be slightly "better" cause even if you don't MAKE IT make it there's still a chance you can enjoy some regional success. And with the internet/streaming + youtube, if you can manage to pop off just enough in your area there's money to be made. You can play local venues, open for bigger acts when they come to town and maybe get some radioplay. There are nikkas in the Bay making money with whole stanbases that I've never even heard of down here.
A clothing line is the longest shot of them all. The fashion industry operates on branding and name recognition. Who's gonna wear your overpriced "orignal collection" when you're a nobody? Even successful rappers and entertainers have started clothing lines that flopped. HARD. The days of FUBU are long gone. It requires a lot more capital to start a clothing line now and you have to pay a lot of people for labor and materials along the way. Designers, tailors/seamstresses, models, fabric/textile suppliers, marketers, etc. And that's even before getting off the ground.