Simply is cheaper than Petit Casino. Petit Casino price are in accordance to the tourism area where it is located while local resident mainly shop at Simply, Leader Price or Super U.
Super U, by the way, located in Marigot where Match was formerly located is a very nice supermarket, with great products and great price (IMHO, the best on the French side) <img src="http://www.traveltalkonline.com/forums/images/graemlins/handshake.gif" alt="" />