I've done it both ways so the question is, what interests you most?
If you want, you can make it a 2 day trip. Book yourself upon the Eurostar and when you get there, go to the Montmarte section, Notre Dame and the Latin Quarters, walk down the Champs d'Elysee and top it off with a visit to the Eiffel Tower (buy your tickets on-line in advance).
Next morning you can visit the Louvre and the Orsay (with or without a guided tour group) and take a late Eurostar back to London.
I love tour groups and have made life-long friends on them but sometimes the tours are a bit "selective" - you should go up the Eiffel Tower and not just drive past it in a bus. The guided tours for the museums, however, are excellent but, again, with a RS podcast or renting the headphones, you will see the treasures of both.