I bought the cheaper version of the cargo area mat from BMW (your link:
http://www.shopbmwusa.com/ProductDet...ProductId=3922). I actually got mine on ebay for about $10 less including shipping. I think it's great for protecting the cargo area carpeting and it has a perfect fit. It has about a 1 inch lip around each side that should protect against melting ice on boots or condensation from a gallon of milk or a bag of ice. But, the side lip is a little flimsy, so if you expect to have a greater amount of ice or water back there, I would go with the more expensive option from BMW which looks to have a much higher and sturdy lip (but also comes at a much higher price).
In addition to the summer (carpet) mats included with the car, my dealer threw in the winter (rubber) floor mats for free, as well as the Y-cable for the iphone. I would wait to buy these until after you pick up your X1 in case your dealer does the same.