I am considering buying a Thule or Yakima roof rack for my Nissan Sentra. I have never had a roof rack, so I don't completely understand the attachment design. I am concerned about theft of the rack - not of the bike, as I will be with the bike at all times. I live in the city, not in a bad area, but nonetheless, it's a concern if I am going to spend upwards of $500. Are these things easy to steal? Are some brands more securely atached than others?