Depends. If you love the zed shape and you have a lot of skills and knowlage about cars etc etc go for the zed but be warned an rb26 zed is not an easy task. its not a car for the faint of heart either. You will need to upgrade brakes, suspension, driveline etc etc
An s15 is a lot easier option and at the end of it all, it is a better, more modern platform to start with.
That said, i owned my RB30det 240z and also an r32 GTR at the same time. I needed to get rid of one to make room, and i sold the GTR. Purely because the zed stands out from the crowd, I love the s30 shape, The zed goes harder, and driving it around knowing that i built it from the ground up gives me a sense of satisfaction.