The cons, obviously, because it is difficult to give an answer to the wrong question. An MD5 hash is a hash of length 128 bits. If you need a 64-bit hash is not MD5. To give some advice on hash, you should know why you need one, because there are cryptographic hashes are common hashes for hash tables, and sometimes hashes are also called checksums. Most likely you need hash tables, but 64 bits is something very much for him.