[spoon]

In stock, there is a bug that gives a slight edge to rock-methane (by taking a little away from ice-methane). ie, the odds of finding a rock-methane planet are slightly better than finding any of the other rock atmospheres (besides rock-none). Thus it gives rock-methane an advantage in the early game, but evens out in the long run (when the odds of finding an ice-methane planet are slightly reduced).

Now, the impact of this is small, but, since I am a play-it-by-the-numbers min-maxer power-gaming munchkin, I almost always take rock methane.

I also consider rock-none if I think the game will be "over" before everyone gets all the colony tech.