Is it your only car? If so it's a bit of a pain in the arse (imo) and I wouldn't go spending more money on one to then have to make do.
We have a 5 door non ST plus two young kids and whilst you can fit a full pram in the boot (sans shelf) and two seats in the back (so long as nobody needs front legroom) I'd be mightily annoyed if I changed car and spent decent money for one only to have to continually compromise.
Nipping out with the full squad in the Focus estate is just a non issue, the fiesta needs a bit of thinking if you're going to have bags etc. I know you already have a fiesta so will know what does / does not fit but it seems a silly change tbh