At the cost of a new RX-8, I'd likely start looking at the 135i too. But, while I admire the RX-8, I wouldn't own one because rotaries are not mainstream tech and is likely harder/pricier to service, and the fuel consumption would be too high for my taste. To the OP, I would stick with the Mazda 3, it may cost more than the used RX-8, but there isn't really a price on peace of mind.
As a side note, I got my MS3 because I wanted a good combination of practicality, price and power. Mazda knew what they were doing when they made this car a turbo FWD hot hatch... nearly all cars are a compromise, and I think Mazda was primarily targeting price vs overall package instead of all-out performance vs price.