I might take the plunge and make Leliana Divine now... so I don't know what I'll think of with Cass' story.
When compared to ideological changes in real world, hardened Leliana seems good enough and believable. I mean some people disagree with her views on mages, but what about everything else? She will follow Justinia's example and is her true heir. You will see this immediately when you encourage her to accept becoming Divine. Do not doubt for a second that she will do what I have quoted in my signature. She will look after the poor, she will allow all races to become Andrastian and join the chantry, she will make the lives of city elves better. if you are worried for security, make Cass rebuild the Seekers.
Cassandra's solutions as Divine works, but its nice to see other perspectives as well. She brings peaces and slight reforms to make the lives of mages at circle better, organize Templars and similar stuff. But one can argue why should we go back to systems that caused chaos in the first place. Shouldn't we try something new? Leliana is a position that neither Cassandra nor Vivienne are. She has a vast network of spies who were on many instances outsmart Corypheus. She knows every secret of every noble. She can bring this change.
Both Leliana and Cassandra acknowledge the corruption and incompetence of chantry and they also recognize how Justinia tried to fix it but in the end she was ultimately alone. Leliana is more willing to walk Justinia's path and permanently fix things.
Of course this is simply a view on things, nothing objective. 