if you use winter tyres then 4 of them, not two - two is just dangerous (and probably illegal / or an issue with insurance)
Winter tyres perform 'better' than summer tyres at temperatures below 5-7 degrees centigrade, as they rubber tends to be a bit softer; so it is not just because of more grip due to profile in snow. Which means with the weather here there's no harm putting them on from now till paddy's day. In my experience, winter tyres are as durable as the normal ones, so makes no difference regarding mileage (however, if profile is below 4mm the tyres are less effective).
I used Michelin (Alpin A4) winter tyres when living in the Alps, as well as Goodyear Ultragrip. Both are excellent, but would be around 150+ euro a piece (depends on size of course)
As I live in Dublin city now I will not bother with winter tyres - main reason is that because of the suboptimal road clearance here you end up with ice anyway and even wintertyres are of no use then. (but I can use public transport or walk and have 10+ years of driving experience in all sorts of snow).