LM Etoile is in a nicer location, but rooms tend to be smaller and a bit nicer appointed. I've not been upgraded to any suites at LM Etoile over 5-6 years of staying once a year.
LM Montparnasse has more suites, and I have been upgraded to a 2-room suite or larger renovated room 4 out of 5 times over the same period.
Either choice would be fine.