We have a Sea Grey 3 door with 2 small childern with no trouble at all both car seats go in loads of space. We took it to Euro Disney last year and it coped with ease.
IMO the 3 doors look better in ST form. Good choice of car I loved your M3 in the for sale thread good luck with the ST. Remember to join the STOC as the London SE memebers have lots of RR days and local meets.