I sold my S15 last month for $33k with 27000kms on the clock. I'm sure I could have gotten more but I thought I'd sell it asap. Not sure what they're going for still but they are coming down in price. The S15 was an awesome car, I would recommend it. However, I didn't mod it but I've heard it responds well to mods. My friend has an R33 and our cars were pretty similar in performance as both were stock. I think the S15 handles a lot better though.
Anyway, in your position, I would get the S15 (if you intend to keep it for a while) as it's newer, less problems, looks slick in my opinion and performs very well. Also, better resale I believe.