Haven't used the Mont Blanc.

I have used the Thule Crossroad 450 with square bars on a 2003 X5, and now on our X3. Works brilliantly. Much cheaper than BMW. Rubber feet, no need to wrap the rails. One key system to secure it. Lots of Thule accessories available for kayaks, etc. I use bike carriers. I like the square rails because they carry a wider variety of accessories. They don't look as sleek as the aero bars, but I have never minded that.

The Crossroad 450 feet appear to have been superceded by the RapidSystem 775 (with square or aero bars) but they look identical in the on-line Thule catalogue. I have always got mine the same day from one of several local Thule dealers.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue