Spot on mate,
You have more or less answered this yourself,
Where the 17" rim has alloy
The 16" rim has tyre side wall
Experts recommend using a 15" or 16" rim for winter tyres because of the 45-55mm sidewall they can be slightly deflated to provide better grip in winter conditions,
Im rolling 18" but I carry a 16" spare which is the same overall radius as the fitted 18"
The facelift fiesta will be exactly the same mechanically as the pre-facelift model brake calipers will be the same etc.
But the zetecS model will have lower suspension compared to other models in the range.
So when rolling the 16" take care on bumps because the have more bounce than the 17" and on bumps the car may dip slightly lower and scuff
Yeah i have been using winter tyres for well over 10 years now, so i have alot of experience with them. Amazing how much difference they make.
I will be using 15" winters in place of the 17" Alloys, i dont have a zetec S, its just a titanium with the Zetec S kit and wheels, so hopefully they will still give me enough clearance.
Although not sure whether to bother changing the 195/50/15 tyres i already have for higher profiule 195/55/15 as they match the 17" more in terms of over diameter