Top of the line Mazda/Honda/Toyota reminds me of buying property. Do you buy the most expensive cookie cutter house in the cheapest location -or- buy the cheaper more custom house in a more expensive location?
I think either vehicle is fine and they check a lot of the same boxes. I might be bias towards noticing more Acuras' and I don't know the yearly sales numbers of Mazda vehicles. I see 20X more 1st/2nd/3rd Gen MDXs around town compared to CX-9 of any model year. Just on my 10 house Cul-de-sac, there are 6 Acuras and zero Mazdas. I see more Rav4, CR-V, and CX-5 in that descending order. I would hate to get a CX-9 to find out 1-2 years down the road why you don't see them on the road.
Its the opposite for me, in my neighborhood. Too many top of the line Hondas, Toyotas, Nissans, Hyundias, Fords, etc., and less cheaper versions of premium brands, although the zip code we live in,, everyone pays up the rear end on yearly taxes. But I still see a boat load of 1st and 3rd gen MDXs in my area compared to 2nd Gens. CX-9s on the other hand, I barely see them, let alone any mazda. A former co-worker of mine drove a mid-00s Mazda6 and kind of destroyed it (lack of maintenance).
Plus once that Turbo goes out, you'd probably be in the 100s in terms of HP and TQ, just saying. Better off sticking with an NA motor if this is a daily or family hauler.