AOKP is a bit more closer to stock in some regards (UI theming for one), but they were the first to really add a crap ton of features to the stock ICS setup. Not only that, but they seem to have a "cult" like following not unlike the early days (and arguably, last year lol) of CyanogenMod. With their ever-expanding list of supported devices, they're certainly staking the claim to be the "heir" of the CM "throne"--that being the one rom to rule them all, or some ish like that.
Having used both (disclaimer: AOKP was the first thing flashed onto my device, I have it now, and have not tried Liquid 1.3), I would say AOKP is a bit more stable. Liquid was the fastest thing out there, but for my device, it seemed to come at a price (some bugs here and there, but hey, it happens and is expected). AOKP is rock solid and as a boatload of options, plus a unique spirit that's kind of cool -- particularly if you love unicorns, rainbows, and excellence.