I would pick the 7th, I love that area of the city. You really aren't going to go "wrong" with either of them, but 7th puts you closer to the seine for the boats, and you can spend so much time just hanging in that area given all the beautiful sights that are there.
We stayed in a studio right near Canal St. Martin too!
It was Feb 2014 when I was pregnant with DD and we had a wonderful time. Super easy to get everywhere via Metro. We also walked a ton & there were bike options as well.
We stayed in the 7th during our last stay and it was great, because we were able to walk so many places. We didn't go to the Louvre this time, but it was right there. We walked to Musee D'Orsay, I think Musee Rodin was also walkable. Walked along the Seine, also very close. Our best dinner was at Le Relais Louis XIII and we walked there and thankfully, back as well, because we needed it lol. Have a great time!