I think the testing done here test situations that actually aren't very helpful/useful. If you know you want to do JUST one of the other then do just what you need to do - right tool for the job and all that.

What you REALLY need to test is the classic UPSERT condition - where you want to update a row by a key if the key already exists and insert the row if the key does NOT exist. There are a number of ways to accomplish this in TSQL and it would be best to test each (although honestly the optimal way changes with the percentage of existing rows that are to be updated). If you do this testing, make sure you build code that is isolated from concurrency issues, which most do not do.