It also depends on things like layover time and time of day. If AMS gave a shorter layover, especially in the morning, I might take the shower there and go to LHR. Otherwise, CDG K lounge isn't "underground"; it's just that the part with natural light (to the left) is small and often crowded.
Time of day also matters. They don't put the champagne out before 11 in Paris. While the lounge breakfast is better in CDG, the difference is not as big as later in the day.