So a couple days ago I purchased a used 2004 S2000, all stock except for Bilstein PSS9 coilovers. Comparing it to the few times I've test driven an FR-S, I'd say there are many, many things in common in both cars (as stated countless times by almost everyone who has driven both). However, what appeals to me about the S2K is its more unadulterated, raw driving feel. It's not about comfort, it's about performance and handling. While the 86 is well-rounded in all areas and makes for one of the best daily driver + weekend car + track tool combinations on the market, I suggest for people looking at a 2nd car to consider the S2000 instead. My daily is an A3 Sportback with a Stage 1 tune and intake, and it works wonderfully for my purposes. Its 2.0T with almost 300 ft*lb torque pulls all 3200+ lb with ease, and its rolling start acceleration is more impressive than the S2000's, making it easy to pass other cars on the freeway. But in the end it's a practical FWD hatchback with low engine sound, a super light clutch and almost no steering feel, and I think I would have regretted it if I got an FR-S as my 2nd "fun" car, since it's just a bit too refined for what I want in a performance car. The S2000 feels more edgy, and that's a good thing for me. That being said, I still gawk every time I see an FR-S/BRZ on the road.
Just my 2 cents.