Uhm shouldn't that be ovious by reading the high level descriptions of each? They are for completely different use cases. Membase is a distributed Key/Value server and LevelDB is a Key/Value library.
I see the difference between a server and a library, but both can often be used for the same use case. Just recently I evaluated a few data stores for a project and I didn't care all that much about the distinction. For the servers you're gonna use an API for your programming language anyway, so the programming model isn't that different.
