My first playthrough, when Cassandra as divine came up, my initial (& authentic) response was: "You'd be perfect!" .... but I still ended up with Leliana as Divine, because as it turns out, Leliana's goals are much more similar to my own.
I think Cassandra is such a wonderful character. She's so fully fleshed--she's both tough and compassionate, believes in herself & questions herself, self-sufficient and a team player. She's the sort of person you hope might really exist in the world, but probably doesn't. I have no qualms about seeing her in a position of power & I'm not surprised she does a good job.
But between a good & conservative leader like Cassandra & a pros-and-cons sort of choice, like Leliana (double-hardened Leliana in particular), I'd still pick the one who shares my agenda. So, Leliana.