From a cold flow (sub-freezing) perspective, M1 0w-40 is still king due to its higher VI. At operating temp, both M1 0w-40 and Castrol 0w-40 are identical, with GC being just slightly thinner at 100 deg C.
Castrol 0w-40 seems to have slightly lower sulphated ash content (1.12) compared to M1 0w-40 (1.3), which could mean fewer additives in Castrol, but not necessarily weaker in terms of performance/protection as there is more to it.
Of course, I'm going by Castrol's European specs which don't necessarily have to be the same as for the product that you guys found on the shelf in the US.