Having had a Zafira as a hire car for a few days once, I'd drive pretty much anything else in preference, it was nasty
Can't really comment on the price with regard to mileage and spec, but your BMW is a lot further down the depreciation curve, so it's likely to be the sensible financial choice, and you know its history. Also, it's probably more economical. Of course, they are quite different cars, and wouldn't normally be considered by the same buyer. Perhaps that could be the stumbling block.