I just traded in my 2012 Civic Si for a 2015 WRX Limited. Here are my thoughts:
-Exterior: My Civic Si was a coupe, one of the reasons I wanted the WRX was 'cause it has 4 doors. I actually liked both, but the WRX definitely looks more like a sports car. The only way you could tell the a Civic Si from a normal Civic coupe was either looking for the Si badge or looking at the rims. Initially I did not liked the WRX too much (comparing to the concept), but it actually looks way better in person. There are some design lines on the body that I could not appreciate in pictures. The same goes for the rims. The rims look awful in pictures, but they actually look nice in person. I am happy with the way the WRX looks over my Civic Si.
-Interior: The interior in my Civic Si was just bad. Even Honda realized this and redesigned the interior for 2013. The WRX interior is a lot nicer. I have seen the newest Civic Si interior, and I still think the WRX looks better. Mine is a Limited, so the leather seats are a nice plus, and very comfortable. The dash of the WRX actually does not look bad; I know some people dislike the fake carbon fiber and the plain radio, but it looks better in person than in pictures. Talking about the radio, I have the base radio and it sounds way better than I expected. I used to own a regular Impreza before my Si, and I can definitely hear an improvement. If you are curious, my radio settings are: Bass 5, Mid 2, Treble 7, with the loud and bass modulation ON, and with the factory Sedan/Wagon settings bypassed. On the other hand, my Civic Si had a a sub on the rear deck, not too powerful but good enough. I would say the Si has a better audio system, but the WRX audio system is better than people give credit for.
-Driving: My Si drove very well. It was a bit under powered, but I did not have a specific complain other than it wasn't the best for driving in snow (therefore I got an AWD Subaru). The Subaru definitely has the power the Civic Si was lacking, but I have noticed that where I could sometimes leave my Civic Si in 3rd during a right hand turn (~15 mph), I need to shift my WRX down to 2nd. I have heard people say there is hardly any turbo lag in the new WRX...well, let me tell you something, the WRX is dead below ~2,300 RPMs. Above ~2,300 RPMs, this thing just takes off! This is something I am still trying to get used to. When we talk about steering and handling, there is no contest. The WRX feels firm and responsive, where the Civic Si just felt soft, and for some reason it felt like you had to steer more than you should just to get the car where you wanted. But I do have one big disappointment from the WRX: the shifter (or gearbox) is very rough. The shifter in my Civic Si was super smooth and the throws were quite short. The WRX has much longer shift throws and there seems to be a lot of play between gears. I get especially annoyed when I am downshifting, lets say from 5th to 4th, and somehow I end up in 6th gear. I am not sure if this is something I am going to get used to. On the other hand I do like the clutch in the WRX better, it is a little bit stiffer (that is just my preference).
There are my 2 cents. Have a great day!