The 16-year-old in me justified a new STi in 2005 with "I never had a chance to buy a car like this when I was younger"; the middle-aged-man in me justified it with "this car is an instant classic, and Subaru will never build a car like this again."
The early STi's are raw, visceral cars, noisy and harsh and tough if you don't like it. Jeremy Clarkson famously said they should have named it the Subaru "you, outside, now."
They demand the driver's attention, and they punish you if you don't give it. Mine had squeaks and rattles on day one - it still does, and I still don't care, 110K miles later. I'm never selling it. I still smile every time I drive it.
Subaru has been softening the edges every year, so now the STi is a lot more liveable in daily use. The differences between the WRX and STi has been diminishing too. In 2005 virtually every engine, drivetrain, suspension, and brake component was different - beefier - on the STi. The wheelbase was longer. The track was wider. I think these days the engines are very close in nature, and the WRX brakes and suspension are closer to the STi too.
My current favorite hotrod Subie looks-wise is the WRX 4-door sedan. Muscular but not brash about it. If I were shopping today I'd drive that and an STi, and then make up my mind. Do I wish I had gotten WRX in 2005 instead? No frackin' way. But I almost bought a WRX wagon (or the Saabaru equivalent) several times in the intervening years. A GD-body WRX wagon is a true do-everything car. Like you, I'm interested to see if Subaru produces a worthy replacement for it, and for the STi.