We stayed in the Marais last year and found it noisy and crowded (though the apartment was fine and quiet). Narrow sidewalks filled with people all going faster than us because they knew what they were doing! ;-) We were only a block from Rue de Rivoli, maybe too close to the action.
Ile St-Louis is beautiful and central, more expensive than most other neighborhoods. Conventional wisdom would suggest a central location, maybe St-Germain, but if you've seen the major sights you might want to branch out, experience a quieter neighborhood, save some money, and let the Metro whisk you where you're going each day. We spent nine nights in an apartment on Rue du Chateau in the 14th, around the corner from the Pernety Metro stop, short walks to the Gare Montparnasse and Rue Daguerre market, lots of restaurants and services within a few blocks. A better choice might be near a Metro stop where two lines intersect so you have more choices for rapid transit. Consider the "teen" arrondissements.