I run Michelin PS3 until November, then switch to Vredestein Wintrac Xtreme, generally until early March. You will have much better performance in ambient temperatures of 7C or less on winter rubber. The compounds stay softer allowing grip to be maintained. 'Summer' tyres rubber becomes very stiff below 7C & can't perform as well. Braking distances are significantly reduced in all road surface conditions on winter tyres in the correct temperature range.
Although you shell out for winter tyres, you can only run one set at a time, so both your summer & winter tyres last twice as long. My Wintrac Xtreme tyres are just starting their third winter having done 9k miles so far and they are still 5-6mm all round.
If the weather is poor (and lets be honest, with the forecast getting chilly) in Scotland, not exactly a rare event, you'll be glad you stuck winter hoops on. Plus pulling cheerfully away from a guy digging his 4x4 out of a foot of snow in a supermarket car park & watching the look on his face as you do so is priceless