Well they are both random except you can buy exotic armor once a month but you still have to earn the strange coins you use to buy them. On the other hand exotics take so much longer to level up all the way where as the raid gear levels up almost all the way just by running the raid again. (And don't forget there are not any exotic boots and with the expansion level 30 will be nothing and exotics only get 30 light maxed out)