I believe the Sport is $1K more than than the sedan, so factor that into your calculcations. The Infiniti may be just $80 more a month, but that's almost an extra $1000 per year. Also factor in the extra fuel cost in the neighbourhood of $500+ a year, as that car will need premium fuel as opposed to regular.
Mazda3 I Sport >> 2016 Mazda Mazda3 - Price, Photos, Reviews & Features

Mazda3 I Sport >> 2016 Mazda Mazda3 - Price, Photos, Reviews & Features

The design of the Mazda 3 was lauded by many as it featured the look and feel of an expensive sport sedan for a smaller cost. This doesn’t come as a surprise, though, since the car is based on the C1 platform of Ford, and it shares the spot with the Ford Focus and the Volvo S40.
Mazda3 I Sport >> The sweet spot: Mazda3 - 2018 Mazda3 range review

Mazda3 I Sport >> The sweet spot: Mazda3 - 2018 Mazda3 range review