On both sites you just start the booking procedure. After you've filled in the date(s) and route(s) you'll be shown a screen with the various possible crossings. There aren't any prices shown, but there are symbols: at SNCM you should look for as many sun symbols as possible (maximum 3), and at Corsica Ferries there are symbols for 'Jackpot', '1 €' etc. for the special offers. On both sites you can go forwards or backwards with the dates to compare offers.