Well, I haven't owned both, and they are very, very different cars. I had a Rabbit, and I was very happy with it:
-very practical
-lots of amenities and luxury car features
-comfortable
-fun to drive
-roomy
But, the gas mileage is not that great; I often got as low as 22mpg in around town driving. The 2.5 liter engine is big and powerful, and the car is very heavy. It's quick, but probably not as quick as the Mazda.
The Mazda, on the other hand, will be more fun in some ways, but is small in a bad way. There's comparatively very little room. It will be more fun to go to a showoff-type event, maybe more sex appeal, but less suitable for grabbing 3 or 4 friends and cruising around, or going places.
Bottom line, I think the Rabbit is an excellent fun and practical car, while the RX8 is a four seat sports car. The tradeoffs are not really competitive but come with the vehicle category.
(If you were to compare comparably, you should compare the Rabbit vs the Mazda 3. Then, the Mazda wins on MPG and outward style, but the Rabbit wins on everything else including comfort and fun.)
