Been there mate .... Got a ZT-T when I got my dog. Now back in a ZS.
It all depends on how much you will actually use the extra space. If you're in the ZS and feel cramped etc , then yes get one. If it's just a "well I think it'd be better", then trust me you won't get any benefit and it'll cost you.
If you decide to go to a ZT-T or 75 , I can recommend them highly. The build quality of my ZT-T was excellent. Really solid car and as a result a far more relaxing drive than the ZS. The diesels go really well with many owners uprating them to touching 170bhp and stump pulling torque whilst still getting 35-40 mpg....or so they claim ! My V6 was swift rather than fast but sounded just as good as the ZS.
They are certainly for the older driver though IMHO. Especially the 75.
My recommendation, stay with a ZS but find someone with a diesel or maybe a 120 who fancies a 180 and do a swap.
If you decide to go with an estate, I'd consider alternatives such as the Passatt TDi sport or Mondeo Zetec S TDCi.
EDIT *
Then also .... Keep the 180 and buy a cheap Scenic / Zafira diesel.