Myself and my wife have lived with a baby a 156 (2000 2.0 TS) and a 159 (2007 2.2JTS Ti) for the last 24 months....
The 159 boot feels bigger than the 156, and the 159 definitely has a bigger 'mouth'. The aircon in the 159 is 5 times better than the non-existent 156 imo.
When loading a baby (now toddler) into the back of the Alfas... the 156 has a higher roof line, which allows more headroom for a parents head! The 159's lower roof caught me a few times on the forehead when trying to put an angry toddler in the rear...
But a small price to pay for an excellent car..
When our baby was 4 months old we packed everything we needed for a 5 day trip away... including a carrycot, Quinny pram, luggage for us... right down to bottle warmers etc... We didn't need roofracks or a trailer....
My wife wanted to upgrade her car in the last few months so we looked at a 159 SW. In the end she went for a 1.9JTDm saloon. The only difference I could see between the saloon and the SW was just more boot 'height'... the car boot itself not any longer/bigger.
We now have a second baby on the way and have no intensions of changing our 159's for a baby/family wagon. The 159's in our opinion satisfy both our family requirements and our will to live!!
If you want any photos of baby carry seats or toddler booster seats in the back of the cars I can post them.
Best of luck with your 159 purchase... go on.. .you know you want one... the fact you have posted in an Alfa forum means you've made up your mind already!!