I would NOT get the hitch mount. Driving in snow with treated roads means a lot of salt/sand on very expensive gear. Not to mention the possibility of binding damage which could lead to binding failure.

Roof rack is second best. Still get some road grit, but less. Road noise an issue. I have driven from Colorado to Boston with a rack (not on a X5) and the noise is definitely very obtrusive.

A good box has less noise and aero issues and keeps the ski/board gear clean and safe. The main issues are low clearance garages and the inconvenience of having to lift skis and boards over your head.

Roof box is best IMHO.