I haven't taken delivery of it tho so in theory I 'could' cancel it, however I won't be.
The way I look at it is that if I am likely to be needing a car for myself at anytime then the Z will be it, if it's a family requirement then the Micra wins. Best of both worlds (as far as my jaded view can see it). One thing I have on my side is that my insurance company won't insure my wife as she hasn't been in the country long enough (happy days) so there won't be any chance of her using the car to nip down the shops. Also my parents live 30 minutes down the road so if there are any problems we can rely on them to help us out with big boot space there anyway.
Honestly mate, having talked long and hard with my wife about this (it was a discussion had both before and after we found out she was expecting), my wife really has no issues with the two of us having a car each, after all there are many families out there who can't afford to do that, we are lucky in that quite simply we can.
If you can afford to do so, get the car.