Both are great fun, but they are also both basically hot versions of econoboxes, and it shows. Given that even a year is a long time to sit in a car, I think the STI is much more livable day to day, and certainly easier to have a conversation in while driving 70 down the highway.
Back ten years ago I think they made much more sense than they do today. Just look at the crap that was out then. The regular GT Mustang was a pile. There were no 135/335s at all. 350Z was new and not all that great. No Genesis Coupe. Etc.
For a go-fast car that's extremely livable, there are a ton of options I'd take over the STI and especially the Evo (reliability and Mitsubishi are almost completely mutually exclusive long term, I've had several DSM Turbo vehicles).
135i even stock, let alone tuned is a pretty gigantic step up. Need more room? 335. Want something that is a bit less german lux, and cheap to maintain? 6MT V6 Mustang, or the righteous 5.0 GT. Even the 370Z, G37 Coupe, Genesis Coupe are great choices.
My tune would be different if I didn't live in Texas. AWD is meaningless 99% of the time here, and the times when you might want it, you can get by driving a tad more carefully.