The advantage of the mirror on top like on a Brunton 15TDCL and copycats is that it can also be used as a signal mirror or for veiwing one's own face (to check on injuries). In survival gear multi-purpose is a good thing.

Its my understanding that the mirrors on the bottom aren't available for such uses without damaging the compass, though I haven't seen one in person in a long while - can't remember if the mirror swings out or not.

I've given the lensatic compasses an honest try, spending the big bucks for the Cammenga, but I just couldn't see why it is preferred over the 15TDCL. They are heavy, bulky, don't have any declination adjustment, and don't have a see-thru capsule (which is another problem with the mirror-on-the-bottom compasses).

I'm sure the military has its good reasons - the idea of a fluidless capsule is interesting, but my own preference is still the Brunton 15TDCL.