I went with the roof rack, those hitch things look dorky and put alot of stress on one single point the hitch. i'm leary as to how much weight you can put on there. and if the weight is not distributed properly (etc.)
I have a family of 7 (including me) so we fill the SE7 I was one of the rare and lucky ones (from what I hear.) to have gotten a flawless NWP rack.
The thing to look at with Roof racks are the styling.
some racks (like mine) have extra clearance so I can still open my moon roof's
which makes is hard to find parking in building if your lucky you might clear a 6'8" structure.
without the extra moon roof clearance 6' 8" shouldnt't be a problem. (watch out for speed bumps in the garages)
as for MPG unless your roof rack is fully loaded all the time.
when not in use it's not going to be an issue (unless your a Nascar racer trying to get that 1/1,000,000 shaved off your time and if you are. you shouldn't have gotten a Disco
)
really it's going to boil down to:
1)do you take that many trips that you would need either?
2)how munch money your willing to spend?
3)how cool or dorky are you willing to look LOL
Hope I helped a bit.
Rudy
