I have an '08 STI and my wife drives an '09 Legacy wagon. We only have 1 child but we do have to pack another toddler around now and then.
'08 STI will work fine, ample cargo space for strollers, kid seats etc but the Legacy has even more room. If you go on a road trip I would recommend a cargo box for the top(thule) to hold extra luggage and golf clubs. STI still has the fun factor about it but it is on a very similar platform as the Legacy.
I would stick with the STI.