Due to the composition and mindset of our group, we found that a co-op game is preferable, as most of the time, the metagaming of most of the guys simply breaks the game, making for a dull experience for the players and much wailing and gnashing of teeth for the GM/DM.
So when we discovered Myth, we knew had found our niche. It pits the players against the game and through some clever rules design, you're forced to keep moving, but you're also better off not barging in like a berzerker on speed, as that would be just as detrimental. Also, the more actions you take, the faster the system can react, so there is a lot of balance.
That being said though, I must admit that playing well thought out campaigns with a DM to giude the flow of the game does feel more organic and interesting, as long as the system is break-proof and the DM can throw up enough challenges...