I've had them mounted in cars facing forward, and I've had them pointing up against a diagonal window. Naturally, this is the worst position, since debris can fall into them. But the truth is that there isn't really that much debris that would fall in unless you're throwing popcorn around inside or something. Dust is heavier in a moving car, but it tends to stick to any surface, no matter whether its facing up or forward. So I can vouch for the durability of the CTS tweeter in a car. And they sound great too.

I haven't had any experience with the durability of Vifa tweeters. My guess is that the thing that's gonna kill them is UV. That tends to dry out paper cones and sun bleach them white. So this might really be a problem with the Vifa's, since they're fully exposed. Then again, it might not. They do sound very nice, so I think you'll be happy with them in that regard.