I use neither...
double stick tape is more secure but harder to transfer from one car to another... velcro is the opposite.
I use a super sticky clay type enhesive. it is used to hold your car winshield in place. can be re-used over and over and stays sticky.
It's black in color..you'll have to check your local body or window repair facility. A roll will last you a lifetime...I've had mine for 10yrs now...and I've only used up 20% of it.