Just to repeat what's already been said, check with your insurer. Mine had no issue with winter tyres, as long as they were manufacturer approved sizes. On the wife's Vectra with its standard 17" wheels, Vauxhall approve a number of 17" or 16" alternatives which the insurers are happy with, so downsizing to an approved 16" wheel and tyre combo made life cheaper. Seat's recommended size winter tyre is the same as the standard specification, so I had no choice but to go for 17" winter tyres; I did speak to a dealer and also to Seat UK's technical people to confirm.
In the end, I bought Sailun winter tyres; far eastern brand, but they're the most popular winter tyre in Canada and north America. Very reasonably priced and I rate them very highly, although not as good as the Continental winters I had on my Audi previously. Then again, they're £100 a tyre less than the Continentals! I'm running the winter tyres on my standard alloys, as the cost of buying steel wheels or a spare set of alloys was prohibitive. Ironically, I was looking for some winter wheels just before looking at your post!
And finally, winter tyres were a revelation when I first got them; my old Audi 80 just ran rings around most 4x4s with their standard tyres. Remember that it's not a license to drive like a nutter; the laws of physics still apply, but handling is just so much better with them and the car feels much more confident with them on.
I'll shut up now!
R.