For me, I get around 120-140" of snow a season so I'm inclined to get snows. I'm not a fan of the ROF GY's that came with the car. They aren't great. They don't give the car the rubber it needs. But...again they aren't bad. I've driven them through some really bad roads (have about 2,200 miles on the car now) and haven't had any problems. With the hp/tq. of the 550, I think dedicated snows, being RFT or non RFT would be a benefit.
Recently in some rain and temps in the low 40's, it didn't seem to have that great of grip--as we all know all-season tires are no season if we boil it down to performance. And the F10 (not trying to start a war here) that the 550 has some power. I think your driving scenarios and where you need to go dictate a lot. I might need to take some all-important road trips to doc appointments out of town that are scheduled 6 months in advance. I might travel ahead of time to get there--but I want snows to help if weather is bad.
I've had RFT Blizzak LM25's on a 328xi and the car was a tank. I don't know if they have any advantages over the Dunlops which BMW is putting together as a "kit or assembly." I've had Dunlop Wintersports before and they were great tires.
I think if you get rare snowfall, that you'll be fine with all-seaon tires if you know how to drive. However with my weather conditions, not having snows is just asking for it. Another plus, is that if I'm going with an OE BMW Part Number, it's covered under my wheel and tire insurance.
12 550xi Dark Graphite Metallic II. Convenience, Cold, Driver's Assistance, Premium Sound, Sport, BMW Apps