I'm also running Viper. I really like the customization options in it, though aside from that, nothing else different in terms of battery or performance vs. NOS. I suppose one would be the headphone jack bug where the Notification Light would stay on after unplugging something from the headphone jack. That's not an issue on Viper.
As for AOSP, I guess it depends on what you want to do in terms of customization, but I would personally grab Cyanogenmod once they have a stable release, as right now it's all nightlies.