Depends if you like riding with the windows up or down. It would also be easy for someone to steal the board (grab it and go) with it hanging out the window. I believe a roof rack offers a little more protection as, if it is secured properly, someone looking for a quick steal may pass it by as they would have to take time to unsecure the board before running off with it -and if it is mounted to a SUV it just adds that much more security as someone would need to climb up and unsecure the board before taking off. Most people looking to steal a board are looking for the grab it and go, and are not going to take time and risk being noticed by climbing up on a SUV and doing whatever to gain access to the board -

I see too many people loosing high dollar boards by leaving them in vehicles with windows down or just tossing them in the back of a truck. Go with the roof rack, secure that board and dont wander too far from your vehicle when making stops to/from the beach.