I have both paid versions of these launchers, and I find they both have similar core functionality (in fact, both of my setups look virtually identical at first glance). However, my preference goes to AL3.
Firstly, average usage for 3 hours on both launchers showed AL to use 29% less memory than Nova (this could however be explained by longer usage on nova), although both ran very smoothly. What really sold me on it was the swipe from left (anywhere), on home screen to launch apps (I disabled swipe left from edge on any screen, as it did not suit my preference). The result, combined with launch animation of flipboard app, gave it an incredibly similar experience to briefing, but with my own personal flipboard, which is profit in my eyes. I have also found that the use of the quickpage feature (basically an active widget drawer), to be pretty useful, if not just for some eye candy.
Like I said, both launchers are fantastic, it's just that Action Launcher 3 has a few extra features that make it operate much better for me! I hope this helps, and good luck with whatever launcher you choose!