There is never a replacement for an STi. I had an 06 STi, and then bought a 2013 WRX. My STi feels just about the same as my WRX granted they are two different generations.
The STi has bulletproof components, while the WRX did not. You pay for the STi's components vs. the WRX's though.
If you have already modded your 06 WRX, I wouldn't buy an STi. I would do some of the upgrades suggested such as brakes and suspension.
13 WRX - Current - Nameless Axelback
03 Audi A4 1.8t SOLD
06 STi - Minor mods SOLD