Having owned several, including the s2000 and the 350z I would say the next logical step is which ever one you enjoy driving the most. Everyone has personal preferences, and you'll find that because this is a 350z forum you'll tend to find most peoples opinions are biased towards the zed. All the ones you mentioned have their benefits and disadvantages. Me personally I found the S2000 the better car for precision, and fun, but the zed was a better get cruiser. The s2000 is very comfortable and the running costs better than the zed. But the zed was better on insurance. Zed was good for torque, stook was good for working the gears. Like I said, go drive them and decide for yourself. With regards to the rx8 I've been looking at them and the rebuild can be done for about 2.2k so if you pick one up for 2k you're still cheaper thana zed or a s2k