I've recently switched from AOKP to Paranoid Android and I'm loving it. In AOKP, when you hold the power button, you get an option on the menu that allows you to hide the navigation bar. In Paranoid Android, you get expanded desktop which hides status bar and navigation bar. What makes PA so cool, though, is you can have certain apps (or your homescreen) automatically go into expanded desktop so you can remove the navigation keys from only the screens you want rather than switching it on and off.
The next cool thing about PA is what they're calling pie. Basically, when you are in expanded desktop mode, you can bring up some on screen navigation by sliding your finger on the edge of the screen. Think expanded controls from the stock browser. It's very slick.
As far as custom ROMs, I'd either recommend AOKP for the extra included options (nav ring shortcuts are amazing) or PA for its hybrid (phone/phablet/tablet) per app customizations. I tried CM very briefly, but switched back to AOKP because it offered more. It's stable and quick. I've noticed AOKP is faster than PA, though, while my phone is underclocked so I might have to stop underclocking on PA.
Another thing, AOKP has custom LED light controls customizable. Awesome ROM.
Sent from my Paranoid Nexus 4