20" tyres are pure bling. They give a harder ride and because an X5 is not a race car are totally pointless other than as 'fashion'. They are more likely to be damaged by potholes, the life of 20" tyres is quite short and they are very expensive to replace. Having got that nonsense out of the way, the sensible route is to go for the 18" wheels and also the winter tyres option on a spare set of 18" wheels. I drove those with Pirelli Sottozero winter tyres through the snow from Austria, via Germany to Farnborough in November last year and they were excellent, even up to their top speed of 130mph. But that was in an F25 X3. You would do yourself a big favour by going for an F25 X3 rather than an X5 anyway. Unless you need 7 seats, the X3 renders the X5 excessive and pointless.