Most recipes I've seen for cannoli use ricotta & sometimes mascarpone as well. Tiramisu generally uses mascarpone, although I've seen recipes that substitute low-fat ricotta. I think they're very similar, although personally I think the ricotta has more of a "cheese-y" taste than the mascarpone. I think you'll like the cake.
Here's a statement about substituting for the mascarpone that may give you an idea about the taste: "* As a substitute for mascarpone cheese, combine 2 (8 ounce) packages cream cheese, 1/3 cup sour cream, and 1/4 cup whipping cream; beat well."