My first aftermarket interior shades used suction cups. I didn't care for them, because the suction cups didn't always "suct" and the shade would fall off. Which kind of sucked. Plus they left round marks on the glass unless I washed the windows after every time I removed the shades.
The exterior shades are good for blocking sunlight. Not so good for privacy— at least when the van is in storage. Being on the outside, anyone who wants to peek inside and see if the van contains any goodies worth stealing can just move the cover out of the way. But fine for camping; a peeping Tom peeling back an exterior windshield cover to see inside is going to be noticed, either by you or your neighbors.
And a caveat, if you normally use a canvas exterior windshield cover, expect to sometimes have to put it away wet if you get rain overnight before you're due to break camp. And expect it to shrink over time if you put it away wet. If it dries while stretched out over the glass it won't shrink nearly as much as it will if it dries while rolled up in your wet bath.