Separate stored procedures for inserts and updates?

Question

I have a table in Microsoft SQL Server. Sometimes I need to update, and sometimes I need to insert. I could write 2 stored procedures:

InsertNewPerson
UpdatePertsonById

But I was thinking to write 1 stored procedure instead (SetPerson) which would do both (if there is an ID, it is an update operation , else insert).

Should I create one stored procedure (only one to maintain) or should I create two different stored procedures?