The Cooper ST-MAXXs are much more aggressive tires - a hybrid all terrain/mud terrain tire, while the BFG KO2s are just an all-terrain tire. A more direct competitor to the KO2 would be Cooper's Discoverer AT3 which
solidly outperforms the BFG KOs. While I've had many sets of KOs, I do not have any experience with the KO2s; with that said, I find the ST-MAXX to be superior on-road (although more tire hum while cornering, due to the MAXX's mud-terrain-like shoulder lugs) and world's better off-road than the KOs ever were. Cooper really nailed it with their magical rubber compound that simply doesn't care how wet the asphalt is, and their engineers somehow managed to make this aggressive tire so quiet. The ST-MAXXs also provide much deeper tread which enhances off-road traction and of course gives a longer life. The KO2s are undoubtedly upgraded over the KOs, so I'm dying to see a comparison between them and the current benchmark, the AT3s. However, I wanted a more offroad-oriented tire so I chose the ST-MAXXs.
As for kojackJKU's warning about the stiffness of the MAXX's sidewall, keep in mind that different sizes of these tires have different load ratings. Most of the ST-MAXXs are load range E, but the 315/70R17 (a.k.a. 35x12.5R17) on my Jeep are load range D and ride just like every other load range D tire I've had on my Jeep, no complaints. You'll have much more of a difference in ride quality from what pressure you run in your tires than what the load rating is. If puncture resistence is high on your list, the KO2 is definitely better than the KO, but I still think it falls below the standard set by the ST-MAXX.