Have a look in your owners manual under "winter driving".
Recommended snow tyres to use is 225/40 R18, wheel size 7.5J x 18 for cars fitted with 18" and 19" rims. Obviously, new 18" rims are required for the performance model.
There is a warning box in the manual saying "Snow tyres should be equivalent in size and type to the vehicles standard tyres". This sounds wrong as how can a snow tyre be an equivalent "type" as the standard "summer" tyre? The tread pattern, softer rubber and block movement required for a winter tyre is totally different to the standard tyre.
I would go for an 18" winter wheel as if you slide into a kerb, you have a little bit more tyre height before hitting the rim.
Our i30 Premium come with 17" rims as standard, but I fit 15" i30 rims with mud & snow tyres, so if it slides and hits a kerb, there is loads of tyre height before a rim is hit, thus possibly saving track control arm damage. Would much rather destroy a tyre than damage steering geometry components. Yes the handling is compromised, but does save the nice rims for the better weather.