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Depends on your agility on snow and your athletic ability. The more proficient you are, the more length you can handle and your pole length is important. Don't use ski poles!! Use adjustable length hiking poles, which can handle your weight. If you're proficient in x-country skiing, you may be able to use larger size, as you will have necessary leg strength to propel uphill travel.
Sal, if you're less than 6' and less than 150 lbs, I suggest the smaller size for you. I also recommend a model that has the BOA bindings, as they are super efficient - and safe. Pole length is also important and I suggest you obtain adjustable hiking poles. Doing so will effect how "upright" your style is, or is not, or if you're carrying a pack. After several seasons or if you're a proficient x-country skier, you probably can handle a size larger, which will require more effort uphill in deeper snow.