> Perhaps there's a nice integration between the two?

They both run in Java, so you can combine Sikuli with Selenium pretty easily [0], so long as you're not doing headless testing. I've played around with it a bit, using Selenium to get coordinates of a container element and then having SikuliX operate just on that region of the screen.

[0]: http://www.softwaretestinghelp.com/sikuli-tutorial-part-2/