The 19-inch tyres on the X-Trail are reasonably deep 225/55 profile and the car rides and drives well on them. The 17-inch are 225/65 which as winter tyres will be better in snow but will flop around more in everyday driving. If you are buying a set of wheels, I'd go for the 17-inch (Alutecs if they do the correct stud fixing and offset) and fit full cold weather tyres, then swap back to the 19-inch with summer tyres in March/April. Had your existing set of 19-inch tyres been worn, then I would have fitted all weather Cross Climates, Weatherproofs or Goodyear Vector 4 Seasons to the existing wheels.