those two cars arent even the same. civic si and the sport impreza. the civic was more power, two-wheel drive, use more fuel (or possibly same in real world) and was the civic even fully loaded? doesn't sound like it, when i last looked at it back in january. the txs is pretty much a civic si with more luxuries. and the base trim starts at 30K.
i cant really answer your question. both cars have about the same space in 4-door trim level. but since you said sport, not even close. the sport has tons of more space than the civic. i mean, if you were happy with the civic si, then you dont have to get the bells and whistles in the limited trim, sport premium is still nice. in the sport premium, you can get heated seats, and windshield de-icer. so the impreza is much more practical. but if you dont need the space, or 4-wheel drive then yea, the civic would be cool. you could save yourself a few thousand dollars. but, i didnt buy the civic si is because i needed it all. i needed as much space as i can, awd, and mileage. i bought my sport premium for 21,200 out the door. and i have never wished i gone back. but your case isn't like mine. i have a S2000 in the garage. and i had to have what i needed.
but with that said. the civic is one heck of a car you can buy for sub 20K. quick(not fast) and reliable. although there is the genesis coup and brz i wouldn't mind looking at either. the genesis came from a brand that is not synonymous with reliability, but they have a 100K warranty or 10 years and they does deny warranty based on tire wear.