Le Marais or left bank?

The Marais is lovely and there are plenty of restaurants and such. Alas most of the charming bakeries and butcher shops are now boutiques selling expensive duds to tourists. It is also an area quite overrun with vacation rentals. It is a great place to stay.

You can’t go wrong with either location. I would look at what’s available during your dates and choose the place you like the best.

Both meet your criteria, but personally, I find the 6th between Blvd. St. Germain & the river more charming than the Marais. Yes, I know tourists stay there (too), but it is charming, full of restaurants, shops and galleries as well as being fairly central to most of what one would want to walk to.

If you have stayed on the left bank & île St Louis before, why not the Marais for a change? You are unlikely to regret it. Just make sure you are not too far from the metro stations, as the "middle" part of the Marais (immediately around the Picasso Museum) can be a-10 minute walk to the metro.

If you are bothered by hordes of tourists (and potentially a lot of noise at night) then I would suggest the left bank, particularly a ways south of the Seine. But there are a number of other factors, such as price, size of lodging, stairs vs elevator, etc.

you should read what Rick has to say about the Marais and watch some Youtube travel videos to see if you think it sounds interesting to you. I'm a St. Germain guy all day long.

We've stayed in both and I'm sure you'd be happy either place. But somehow we find ourselves coming back to the 6th again and again.