18th Arrondissement - Specifically Rue Marx Dormoy

You don't say where on Rue Marx Dormoy the apartment in question is located but IMO the area north of Stalingrad metro and between the train tracks of Gare d Nord and Gare du l'est is pretty gritty. We have a friend who lives near this street and we've been to his apartment and restaurents in this narrow corridor and I'm not sure I'd want to be based for 8 months in this location. We've stayed for a month nearby in the 19e and liked that a lot.

The apartment closer to UNESCO is definitely in a potentially "quieter" area. It's closer to the Seine, Eiffel Tower, Invalids, etc. it has some pockets of restaurants and isn't teaming with people 24/7. We've stayed in this neighborhood and I'd feel comfortable staying there again.

Because I don't know the exact location of either apartment though it's really hard to say. One thing I do when where contemplating which location will we choose for our 1 month stay in a European city is to study google maps satellite view looking for markets, cafes, parks, etc. I also "walk" the streets in our potential neighborhood (if available) on google street view to see how I like the atmosphere and nearby services. Sometimes the camera goes down the street at crack of dawn hours though so you don't always get an idea of what the neighborhood looks like when metal doors are up and shops are open for instance. Study the 2 areas and see if you get a feel for the areas specifically on and near the 2 apartments.

Everywhere in Paris is close to a pretty useful Merto station so getting to things you'll want to do in the city will be easy no matter which apartment you choose.

Given the choice between the two areas that you mention, I would choose the apartment near the UNESCO headquarters. I imagine that the apartment on Marx Dormoy is the larger and nicer one. It would have to be for me to even consider staying there. If it is not, unless you plan to take north and east bound SNCF trains all the time and so need to be near Gare du Nord and Gare de l'Est, the apartment near the UNESCO headquarters is just about a no-brainer for me. Much of Rue Marx Dormoy is gritty -- that being said, I have not been anywhere in Paris where I felt concerned for my safety but most of Rue Marx Dormoy is not where I would choose to stay for eight months. I like stepping out of my place on to a quiet street and then making my way to the hustle and bustle so over near the UNESCO headquarters fits better into my personal style. But, of course, you may prefer otherwise. I may be partial but I have spent a lot of time in the area near La Motte-Picquet-Grenelle metro station (about 10 minutes walk away from the UNESCO building) and I find that area to be great for families. Also, there are more metro connections in the area around the UNESCO headquarters than there are on Rue Marx Dormoy. Is there a need to commute to work while in Paris? If so, is one apartment more convenient for that?

I have walked around a bit recently near Marx Dormoy and it is a decidedly down scale neighborhood. It is also a goodly ways out from the center and cultural icons not that with 8 months this is a big deal.

I would pick the apartment in the 7th.

The advice to check out both areas on Google Street maps is a good one.