Test drive them both, although I would go Camry, myself.

Realise they’re both brand new, so their long term reliability is still unknown.

All the best!

Go with the Camry. Mazda 6 2021 has the Cylinder Deactivation for lower trim, higher trim has a turbo in it, but realize its 2.5 turbo, so if the turbo is down you still can drive the car. 

(Someone confirm this) but I believe the 2021 Mazda6 has dropped cylinder deactivation on their 2.5L naturally aspirated I4 engine;  I used to see it listed on their brochures from 2018-2020, but in their 2021 brochure they no longer list that technology (unless they are being sneaky all of a sudden).  Also, there were mentions of it on the Mazda forums that they are dropping cylinder deactivation (also on some other models as well) - but I don’t have confirmation that that’s 100% true.  Anyhow, you mention 2021 model year and those 2 trims - if it were me and assuming the cylinder deactivation was indeed dropped on the Mazda I would probably still go with the Toyota Camry but the Mazda would be right behind it (close second), acknowledging that the 2.5L naturally aspirated I4 engine in the Camry (Dynamic Force Engine) is still fairly new and same goes with the 8-speed AT, so jury is out on the longevity of both, whereas the Mazda 2.5L NA engine and 6-speed AT has been around longer and more true/tried and holding steady.  But I am still giving some faith to Toyota that they still got what it takes to build long lasting vehicles (but maybe no longer as long lasting as they used to - time will tell).

Either way, and again assuming the cylinder deactivation has been dropped in the Mazda (someone confirm!) you can’t go wrong with either one.

Definitely the Camry XSE for me.

Get the Camry. Have a friend with a 2021 SE model and she loves it.

Both great cars. Buy what you like to drive better.