I would not choose either of these two speakers at this point. They are just too old. Getting either fixed would be very difficult. If I had to choose an old planar type speaker, It would be a Martin Logan SL3. Getting parts would be much easier if there is a problem. The SL3 is also an excellent sounding speaker especially for what you can buy a pair for.
I owned Apogee Centaur Major back in the early 1990's and they were nice speakers. The Slant 6 I thought were a bit better and played louder. The SL3s I owned later were better sounding than both. I might also consider a pair of older Magnepan speakers.