We have the up to 18 pounders. Toby is 5.5 pounds and Lola is 3 pounds when she's lucky.
They both have plenty of room to lay down, sprawl out, and still have blankets and toys in there. I would say with you having a Mazda, the 18 pounder in the backseat would be your best bet. Our second vehicle is a Toyota Camry and we can put both seats in the back and still have some space between them.