If you are wanting to see more specific sites such as The Louvre etc then I would definately make it a stop off before DLP. The Paris sightseeing buses are far more accomodating and you can get off and on wherever and whenever you like (and are cheaper too). Paris is not really a city where a short tour does it real justice. The HNY tour completely misses out so much like Montmartre, Sacre Cour, Arc De Triomphe etc. Far better to take that little extra time to explore it's unique magic.
I have been to Paris many times and am still finding new things to see.