They both will last you for 8-10 years as long as you take care of them. The Mazda 3 has a more aggressive look than a wimpy looking civic. You need to decide which options you like more and see which car offers that.
Do you wonder why they call it "forever"? You can use a forever stamp forever. 20 years from now when postage is $10, you can still use it. That probably was why they named it "forever". The new forever stamps still have the Liberty Bell - it just costs more to buy them now.