For me personally...I couldn't get the Z because I just can't deal with the Pontiac Sunfire esq interior and how rough around the edges they are. No back seats, shitty switch gear, and just nothing interesting or sexy inside. Also, I hate the stereo type they have at the track..tons of power, tons of tire, yet there's 5 of them getting passed by an E30 or a Miata. Don't get me wrong they're fast and with a proper set of marbles can be sick track cars but it's something about how they are set up, whether it's their chassy or just how they feel in general that bores me everytime I drive one (driven the piss out of a C6Z, LS2 Z51 and 98 LS1). However, their sound and outright straight line speed almost out weighs this.

As far as the Sti goes I just can't take them seriously unless they are in a soft or dark color, wingless and just plain as "un-poser boy" as possible..Don't get me wrong i'd love to jump in one at any road course, however look how extreme track cars look compared to street cars. They're more at home looking there despite the fact that they are as aerodynamic as a brick. Get my point?
On that note, I assume you have 23-30k to spend. I'd have to get an E46 M3 simply because it offers everything, has the best motor and based on my experience is the MOST fun to drive. You can't get enough of those things on the street and especially on the track. However, i'd also consider CTS-V's (although I know the axel hop would steer me away),E39 M5's, B6 S4's (even with limited mod ability you can make them very nice), MK4 Supra's and R33 Skylines.