Get the BMW one or keep the cash and pay for the bits that fall off.
Wouldn't waste my time with any independent insurers personally - IMO much more likely to try and weasel out of claims. With the BMW backed warranty it's all down to how the dealer presents the claim (and therefore possibly(?) your relationship with said dealer). As a matter of interest though, for the sake of a balanced view, has anyone any positive experience from third party insurers like warranty direct?