how much dough are you looking on spending?, what are you looking for in a car? and what do you want to get out of it?

 

Hurray Google!

First Drive: 2006 Subaru Impreza WRX and WRX STI

2006 Subaru Impreza WRX and STi spec page

 

Get the STI if you can possibly afford it. Even though the appearances maybe similiar, stock vs. stock, it's a completely different vehicle.

06 STI over a 06 WRX differences (est. price in quotes to add it later to a WRX, not including labor):

- Bigger turbo ($300-$900 depending on used vs new)
- Better engine (~$5k, maybe more)
- Better brakes ($3-4K for a new Brembo BBK)
- Stronger tranny ($2500 to $5000 depending on condition)
- DCCD adjustability (add cost to the above)
- Front/Rear LSD (???, but probably $1K)
- Improved Suspension (another $1K for sways and springs, but the STI still has a lot of better parts)
- Higher priced rims (wider too) ($600+ a piece new, $800-1200 used)
- Upgraded interior panels and seats ($800 to $1600 used)
- Shift console trim ($75)
- HID, adjustable headlights (expensive... $1000+ just for used)
- Intercooler spray system ($150)
- Larger Wing (could be free if you find someone to swap trunks, otherwise $700)
- Sidebody strakes ($300-400)
- Rear fender molding (can't find for WRX)
- STI Badging ($100)
- Underbody diffuser panels ($500, custom fit on WRX)

I know I'm missing a bunch other stuff...

Plus there is hidden costs (like tuning, supporting parts, attaching hardware, labor, etc)
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Let's not scare the dude off of a WRX if he doesn't want to spend the extra cash on stuff that doesn't have anything to do with performance. I've taken the liberty of making a couple of changes to your list, hope you don't mind.

Get the STI if you can possibly afford it. Even though the appearances maybe similiar, stock vs. stock, it's a completely different vehicle.

06 STI over a 06 WRX differences (est. price in quotes to add it later to a WRX, not including labor):

- Bigger turbo ($300-$900 depending on used vs new)
- Better engine (~$5k, maybe more) engine is almost the same, no need to upgrade, really
- Better brakes ($3-4K for a new Brembo BBK)yes, the Brembos are better but the '06 already comes with 4-pot fronts so better pads, fluid and lines go a long way to improve the stock brakes
- Stronger tranny ($2500 to $5000 depending on condition)
- DCCD adjustability (add cost to the above)
- Front/Rear LSD (???, but probably $1K)not really necessary unless you're planning on serious track time
- Improved Suspension (another $1K for sways and springs, but the STI still has a lot of better parts)if you spend $1k on sways and springs then you're shopping in the wrong place, for that kind of money you're close to coilovers which would put the STI suspension to shame
- Higher priced rims (wider too) ($600+ a piece new, $800-1200 used)no need to upgrade the wheels either, 225/45/17s fit just nicely, which is the stock STI size anyway
- Upgraded interior panels and seats ($800 to $1600 used)seats may be nice but otherwise useless
- Shift console trim ($75)short shifter - maybe, trim - not so much
- HID, adjustable headlights (expensive... $1000+ just for used)wouldn't call it a performance enhancer
- Intercooler spray system ($150)useless, if you're having bad enough heat soak issues then just upgrade your TMIC
- Larger Wing (could be free if you find someone to swap trunks, otherwise $700)this is a personal taste item, added downforce is only really relevant at too high speeds anyway
- Sidebody strakes ($300-400)not a performance adder
- Rear fender molding (can't find for WRX)not a performance adder
- STI Badging ($100)not a performance adder
- Underbody diffuser panels ($500, custom fit on WRX)adds a bit of high-speed stability but not really necessary

I know I'm missing a bunch other stuff...

Plus there is hidden costs (like tuning, supporting parts, attaching hardware, labor, etc)
-

IMO - the only real benefits to the STI are the transmission, slightly larger turbo, and suspension. The latter two are replaced by most owners anyway.

Bottom line, STI is the better car between the two in stock form, but one should be realistic about one's goals and understand what is and is not important on the car when it comes to performance. As far as appearance is concerned it's all subjective anyway, so the OP can make up his own mind if a particular aesthetic feature of the STI (ie. the park bench wing) is worth the extra cash or not.

 

^ Oh I agree, but I'm only concentrating on the differences, not necessarily if the differences are worth it. And the differences in terms of physical hardware are significant.

If you really want an STI to begin with, your better off buying up front, rather than trying to create one...