Have to disagree with Blvd. de Grenelle, which is incidentally in the 15th and the métro (overground) is going through. Pretty noisy and not the most charming street. However, the métro stop, La Motte Picquet-Grenelle is pretty convenient.
Rue de Grenelle (7th arr.) is a different matter and a much nicer street. But, alas, that's not where the Citadines is located.
I'd check on Tripadvisor what people who have actually stayed at the different Citadines have to say. Again, a lot of reading for you, Madlyn, but I think, it's worth the effort.
http://www.tripadvisor.com/Search?returnTo...=12&submit.y=14Regarding location, I'd go with the Opéra-Vendôme, Grands Boulevards or Louvre.
My fave however, is the Saint-Germain (Grands Augustins). There is one bad review though. But, oh well, some are good. What to believe??
Some of the others are in a good location too, but I wouldn't hit them first, especially, if you're not familiar with the city.
Les Halles is to be avoided.