Would recommend the 5 door.
You say your young'un will be 15 months so in a forward seat or soon to be. As well as that mentioned such as access in an accident etc then also remember that the rear window does not wind down and the windonw line is higher. With kids, car sickness is always to be considered . (yeah for leather seats) especially with Type S 'sporty driving'

The idea of draging such an event out past the front seats! especailly if like me you take them out in the seat to prevent further drips

or not being able to open the back window to give then some fresh air and that higher waste line restricting the view which can induce sickness (even I feel closed in in the back of a 3dr). Add to that the practicality of truely magic seats. And remember you will probably still have the car when she is 4-5 years old. Would a 3 dr still be practical then?
For me it would be a 5 dr. Good luck.