Chez Michel and Chez Casimir both share the same kitchen and both are run by Thierry Breton. The two places are not the same, though they do share an emphasis on fresh, quality produce. Chez Michel is more "bistronomic" and Chez Casimir is more simple bistrot fare with less "prestigious" produce on the menu. There have been changes this year, Chez Michel now has a big bread oven for slow cooking dishes and a 50 euro menu (with supplements), Chez Casimir now has a 32 euro menu (also with supplements for higher-end produce). Good, very affordable wine list. I haven't been back to Chez Michel since the price hike (the menu was previously 32 euros), but I've had many good meals there before. Now you'll be looking at upwards of 65 - 80 euros / person. In the 10th arrondissement, that's not really in the price range for a "regular" bistrot. And in the 50 euro price range in Paris, competition is tough...
What I heartily recommend though is going to Chez Casimir on the weekend for the "Traou Mad" (which means 'Good things' in Breton), when there is both a buffet of cold salads and hot dishes served throughout the day. It's a great "French" ... um, Breton... brunch option