Hi Jeff,
We have played around with both Ribbon and Planer-magnetic tweeters but have not found any good reason to consider using them. Maximum output before compression can be a problem with these designs and the off axis response lacks uniformity. If you are in the camp of wanting your tweeter to operate out to 40kHz and beyond then a Ribbon tweeter crossed over as a Super-tweeter would be a good thing to do. I have run tests with this in the past and did not find any reason to be concerned with the response out to 40kHz.
