First off, I don't own a rod rack for my vehicle. I've thought about it too. But, here is my take.

I think a magnetic mount would be more secure however you have the risk of scratching the paint. However, that's a mute point if you're fishing in your Corvette.

I like the suction cup mount, and I think that would be the way to go. You can stick the mount on your hood and windshield. Handy if you drive a truck. I found this company that would interest me. I've been assured a number of times they won't fall off. However, I'm sure there are failures due to human error.

Sumo Rod Mount