Adding on to what others have mentioned, besides the EJ257 having DAVCS/AVLS, bigger intercooler, better suspension/brakes/transmission/differentials (your wrx is all open diff), you have the same car. Fueling is actually the same (5-10cc difference) and the STi will require fuel mods to be reliable with anything above stock power levels (research STi fuel stumble). Yes the STi will keep its value longer but you pay for that every month, and can get walked by a built wrx with the same turbo.
Typically unless you meet one of the following requirements, you don't really need the STi:
Track ready, 6MT, full LSD drivetrain, powerband in higher RPMs, status/bragging rights, resale value, or larger part selection (only compared to the EJ205/FA20.
It sounds like you may very much enjoy an STi over your wrx, but unless you can get a great deal on a clean one and haven't already invested lots of money in performance mods I wouldn't bother. While you can do a bit more with a built STi vs a built wrx, the cost of doing so will go up due to having more parts to work with.
I own a 02 wrx, built block, bored/stroked, JDM Type RA 5MT, clutch/flywheel, suspension, full exhaust, and running a base map...I still keep up with a stock STi (okay 1/2 car behind). I drove a 2016 STi and got on it hard, I will say while those extra features are dank af, I can live without them. I'm not a fan of having to turn off all the safety features (VSC/TCS) just to have fun, and the feel was totally different from my GG chassis. That car STOCK is more than enough to get you arrested if you don't behave, it's also "almost" enough to out run most 5-0 cruisers but that's not why you're here lol.
Conclusion, if you have an intake/TBE and stage 2 yet find yourself wanting more..find someone who will let you drive theirs (good luck haha). The DCCD is one reason on its own to get an STi but only if you find yourself hooning off the tarmac/pavement more than you are on the streets, it also requires more maintenance if you utilize it often.