I just purchased a 2015 WRX as a weekend car. I still have my 2013 Si to drive to work and am using that to commute about 500 miles per week. I love both cars but they are very different. Here is my list of major distinctions so far.
Engine- the civic si is very smooth and has a good sound but cannot compare to the pull of the wrx. Also love the turbo noise. Advantage WRX
Steering- This is not even close. After I drove my Wrx for the first time, I got in my civic and thought the steering linkage was broken. It felt loose and sloppy. Advantage WRX
Trans- I like the WRX clutch but Si is so smooth. Si wins this one
Radio- not even close. The WRX sound like it has at least a couple blown speakers. Si radio wins by a mile.
Styling- In person I feel the WRX looks better. I like that it is sporty but also looks like a sleeper from far away.
If I could choose only one it would be the wrx. The major reason is the handling and the power it has compared to the si.