[urq] Mud and Snow or not?

[SpotatAshleys]

What's the general concensus on the wisdom of fitting my tatty but straight 6" ronals with mud and snow tyres for the winter (UK) in place of  my pristine 8" ronals? 
Any drawbacks with M&S tyres apart from road noise? Would normal road tyres be a better bet?
I haven't fitted any tyres to the rims yet. Just thought I'd make use of any previous experience.

Many thanks

Keith

'87 WR

p.s. can anyone give me a link to details of the biggest/best brakes that can be fitted to a WR whilst retaining 15" wheels? (currently has G60 calipers). Cheers