I, too, was initially concerned about engine, wind and tire noise on my 2016 GT. Happy to say that I am pleasantly surprised. But I also quickly swapped out the OE Dunlop tires for 19' Michelin Pilot Sport AS3 tires and they vastly improved ride, comfort and handling. (Spoiler alert: I work for Michelin.)
As for why Altima sells more, it's probably advertising, dealer locations and comfort with the Nissan brand. Mazda has a bit of maverick in its DNA and so it isn't as mainstream as Datsun?, whoops, Nissan! My 2 cents...
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