I haven't tested this but I think the merge will be slower. A merge will check the merge conditions which will take extra time compared to the blind append.

So unless delta lake has a check built in to check on empty tables, my guess is it will be slower.

By how much? No idea, I don't think it will be by a lot. But a blind append is of course wicked fast.

If you want a single script for init/incremental, you can do a check on a count of records in the target table. If that is 0, pass a different query.

I like to keep them separated though.