So I take it you can't put the spare in the trunk? If that's the case, then I would say to get the hitch mount. Honestly, the roof rack solution was not bad at 80-95 mph. Everything is going to sound loud at 120.
I have been carrying it the rear compartment but there always seems to be something else to haul, then it is in the way. Will try it up on top on my next trip if you say the noise is not bad under 100. How are you securing up there? Can you foward on a photo?