Leona Lewis I think is going to keep selling. She's got the young crowd hooked with "Bleeding Love" on pop radio, and I think it will grow on the Adult Contemporary stations...songs slowly build on them.
I see Duffy pulling a KT Tunstall. A lot of curious people who like her first or single, but don't hold onto her (see: Drastic Fantastic). She'll stay solid, but only her dedicated fanbase will keep following her.