You can use union i think but you must merge vertex again... i suggest you to use combine and merge vetrex it's the cleaner ( im noob 2 .. about 1 - 2 months practise but i use this method )

And Boleons are smarter way to combine but when you do it ... it get reall bad vertex all around the 2 objects ... i prefere the Combine method it's a matter of Style anyway ...