Congratulations :P
in all honesty either of those models is a good choice, if you can run to a newer model with a 2.5 turbo that has more lower down torque which gives a very civilised drive - also we have managed no problem with a hatchback and I know people who manage with kids and the saloon.
Why not try driving each of the models, imp saloon, wagon and forester to see which suits - when it comes to kiddies obviously safety is paramount so the wider track and bigger brakes of the STI aiding stability and stopping with confidence are clearly a requirement ;)