I've now installed WordFence on a few sites and it and Sucuri are quite complimentary. While they do duplicate some functionality, WordFence appears to be more feature rich (though most of the extra features are available in free plugins) while Sucuri wins out on the human element.
I think I'll run both and keep Securi as my paid option (I'm stuck with them for a year anyways, and like the peace of mind of having a security expert on call).
WordFence is a great login auditing/blocking tool, and I love that it can block bad crawlers with one click (which can quickly spike resources on a VPS as they generally don't rate limit requests) and makes finding and blocking bad bots/heavy resource consumers easy as well.
_________________________