You've probably had this conversation with yourself as an artist. Buying exclusive beats is always the best step in advancing your career as an artist. No one else can purchase the beat after you buy exclusive rights. You can generate income from royalties with no cap to stop your profits.
Leasings is a good option also. Leasing beats works great with putting together a mixtape or working on a demo project. If your working on a single, you may want to upgrade to exclusive rights in the future. If the song blows up and he's already sold the beat then you are in a tough situation.