Remember that with any used car, you are in a sense buying the previous owner, their care level, and the car's maintenance history.
Remember also that lower mileage does not necessarily, by itself, indicate lower maintenance cost over the coming several years. It might in this case, or it might not. E.g. if you are looking at some arbitrary car that requires an expensive timing belt/water pump service and 100k miles, an example with 120k miles that has already had this work performed will, ceteris paribus, cost you less than a 90k mile example that hasn't.
In the context of your options, this is to say: find out as much as you can about the Civic's service requirements for the next 50k (or whatever) miles and what is either certain or likely to be needed on this particular car.
Also, what is the basis for the $$ you foresee spending on the Mazda? Is it based on the performance of the car itself (e.g. the suspension is worn out), or a sense that "well, this must be coming" ?