I'd use GC then. There's also Castrol 0W-40 A3/B4 which is a newer product which you might consider.
All these oils meet Mercedes 229.5, a very stringent specification so the performance will be similar. NVH becomes a greater determining factor. M1 0W-40 probably gives the best performance, but it does tend to produce more noise and for that reason my father has PUE in his X5.