MsgPack is simpler to implement, and thus someone needing to implement it, is going to have a better time.
CBOR is an IETF standard, meaning that CoAP has a content-format (A.K.A a number telling what is the format of the contents, like if they are in JSON, XML ...)for CBOR (for deflated CBOR, acutally) but not for MsgPack.
The two are very close (indeed CBOR is a non-welcomed fork of MsgPack). Their differences are: