The RX-8 is junk. They look cool, but cost a lot to maintain and repair and are not fast at all. They get terrible fuel economy (around 24 on the highway) out of their small 1.3 liter motor and eat oil continuosly. The rotary engine is worthless as well. They are good for abotu 60,000 miles before they grenade
If the Mustang is a GT then it is a toss up between the Mustang and Infinity. Are you more interested in a muscle car or a luxury car. The Mustang will be much cheaper and easier to repair and maintain than the Infinity (and is faster), but will get worse fuel mileage and will cost more to insure.