I don't think you need to worry too much about the differences between winter tyres. The fact you are using winter tyres of the standards of the above tyres is enough really. I mean, its not like you'll be booting it about in the snow and ice.
Relatively slow, safe speeds will negate pretty much most of the differences between mid - high quality winter tyres.
This winter will be my first winter in a BMW (or any RWD motor), and I have a set of 235/40/18's to go all round on my standard MV3's. I know its a staggered wheel set-up with non-staggered tyres, but then they were cheap at £170 for a set of Conti's with 6+mm. I also have a pair of brand new Bridgestone 225/40/18 RFT winters too, just in case.
And as if all that still doesn't cut it, I bought a Range Rover just to be sure, lol.
530d M-Sport Touring.