From my understanding, BMW Roadside assistance uses whoever is closest to your location. Having had my car towed twice, both ended up being CAA flatbeds, both times I waited 2-3 hours for the tow, fortunately for me, I was not on the highway.
I have had good experience with Abrams, although they could not provide a flatbed when I called, they were the first company that managed to get someone over with a service truck to assess the issue and manged to get the siezed wheel off the car, so we didn't need a tow that time.
CAA is a better service provider for tows however.
MY07 A22 E90 N52Nxi