I own and s2k and have driven rx8s.
RX8 is no where near as nimble feeling, ride is heaps softer, but is more quiet and better comfort.
official L/100km is 12.6, but it is way higher. When fanging it, fuel goes down the drain. Expect as even as low as 200km from a full tank of hard driving!
Also youre required to top up engine oil quite often (like litres) - i think even between services - it actually says that as part of the manual.

It also looks crap in my opinion. The motor sounds so unexciting - big hair dryer. Gearing is also really tall.

Then again i can see why some people buy it. 4 seats ("4 doors") and is comfortable, yet still is quite sporty. It is just a good all rounder, a compromised sports car. Not a choice if you want / used to s2000 sportiness.