Merge faster than Insert
MERGE has more flexible OUTPUT. OUTPUT can refer to the merge source which is handy if you want the client to be able to match what it sent to what was actually inserted (e.g. IDENTITY values). INSERT can't do that (for no fundamental reasons; seems to be not implemented).

I can't think of any performance difference. The plans certainly look so similar that I can't make out a difference right now. If there was a major difference SQL Server could optimize one form to the other.