Why the 6?
Do a Google search of Mazda 3 vs 6 and you'll find half a dozen Mazda dealerships all explaining the same 5 differences, among the many, many differences between the two cars.
They'll start with the mileage: the 3 gets better MPG. Then they'll show that the 3 has 96 cubic feet of passenger volume while the 6 has 99 cubic feet. The 3 has like 12 cubic feet of trunk space while the 6 has around 16. The 6 has a few more inches of rear seat leg room. The 6 also comes with a 2.5L engine standard.
And that's it, according to them. Apparently no salesperson had the time to spend more then 3.5 minutes writing an article to explain the nuanced differences between the two cars. If you were just looking at those articles, you'd think all you were getting for a $3k-5k price difference is a 4% increase in cabin space and a bigger trunk.
So I did my own comparison of the two cars. I looked the 2016 models, both in the Sport trim, though I'm sure many of these carry over between years and between trims. Here is what I found:
*The 6 is roomier inside overall (those 3 cubic feet make a huge difference in making the cabin wider and longer)
*The 6's trunk has storage trays on the sides
*The 6s mirror adjustment knobs and AC adjustment knobs are chrome, the 3s are black plastic
*On the 6, all 4 windows have auto up/down, while on the 3 only the driver window has it
*The 6 has a headliner sunglasses holder (the 3 Touring adds this feature but also adds a sunroof which takes up a full inch of front seat headroom)
*The 6 has an electronic parking brake (the 2017 3 adds this feature)
*The 6 has a sliding front cup holder cover (the 2017 3 adds this feature)
*The 6 has a fold out armrest in the back seat w/cupholders
*The 6 has lumbar support
*The 6 has a flat black window control panel with aluminum accents while the 3 has carbon fiber with glossy black accents
*The 6 has more aluminum accenting on dash
*The 6 has a leather wrapped steering wheel
*The 6 has better looking trim around infotainment screen (the 2017 3 adopted this better-flowing trim)
*The 6 has a larger instrument cluster with a needle Tachometer, while the 3 has a digital tachometer
*The 6 has a CD player
*On the 6, the usb port is inside the armrest compartment, the 3 has it below AC controls in plain sight
*The 6 exterior has a better shape in my opinion, and it has chrome accents around the windows and across the trunk
*The 6 has turn signal side mirrors
*The 6 has 17" aluminum rims while the 3 has 16" steel hubs w/hubcaps
*The 6 has dual chrome-tipped exhaust and I couldn't even spot the exhaust on the 3
*The 6 has speed control on every vent while the 3 only has it on outer 2 vents
*The 6 has 2 individual rear seat map lights while the 3 has 1 dome light
*The 6 has a trunk button on key fob
*The 6 has soft side door armrests in the back seats while the 3 has hard plastic ones
These were all the differences I could find between the two cars, and to me they make all the difference, but feel free to add more I may have missed. I think the 3 would still be a great car to have, I just think the 6 takes all the great features of the 3 and makes them a bit nicer.