Good luck finding a nice spot on the Italian coast with no tourists.
Maybe you won't find North Americans (unless you go to the Cinque Terre where there are thousands of Americans), but you can always count on European tourists (including Italians) being everywhere along the nice places at the seaside.
Some advice on both, since I know both well.
MARCHE: The best portion of the Marche coast, IMO, is the Riviera del Conero, near the Mount Conero Park, just south of Ancona. Numana and Portonovo are both nice little towns and the sea spectacular.
MAREMMA: This is the best section of Tuscany's coast, if you exclude the islands (Elba, Giglio etc.). The top of the top, IMO, is the Monte Argentario Peninsula around Porto Santo Stefano and Porto Ercole. From Porto Santo Stefano, you can also easily take a ferry boat across to the Giglio Island, which is a beautiful island. Just make sure Capt. Schettino of the Costa Concordia is not at the helm of your ferry :-)
Also nice, more to the north is Castiglion Della Pescaia and also and Punta Ala, which is another top area that was developed a few decades ago (lots of upscale timeshares in Punta Ala). I spent some vacations there in youth where I friend had a house (Roger Moore, the former James Bond in the 1970's was her neighbor, he also had a beach house at Castiglione).
Between Argentario and Castiglione you also have another area, the Maremma Park (used to be called Uccellina National Park in my days) with the town Talamone. If you are interested in nature walks and bird watching, you'll like that park.
I'm not giving you advice on which one to choose, since all are nice choices. I can only suggest that you do a Google Image search and see which of the above you might like better.