Interesting question! I have a BSM 1er with Terracotta (and M-Sport package, so I have the black headliner). I really like it, and have gotten numerous compliments on the combination.
I have daydreamed ahead to my next car, and I will say that LeMans Blue is romancing me.
I think that it looks better with the Oyster interior than Terracotta, at least on the configerator.
However, I am the only person in the car 95% of the time, and my back seat has only been sat on three times. I don't have to do much to keep it looking good.
Sorry to stir the pot! However, I'll be interested to see how this thread goes.
IMO, there is no "bad" choice here. Either one would work.