Stay by Montparnasse in 6th or 11th by Voltaire?

We are a young couple staying in Paris for 11 days at the end of august.

Trying to choose between a spot to stay in the 11th near Voltaire station with terrace

Or a place by Montparnasse station with street windows.

Looking to have nice walks in day and night, go to cafes, visit some good easy access restaurants near by, see some night life and visit some good sites. Which spot would you recommend?