Ultherapy or Exilis for slightly sagging arms and knees? (photos)

I am active, however at 63have developed sag in my upper arms and top of my knees. I went to a Nurse Practitioner, who performs both at a Plastic Surgeons office. I went thinking I would being a candidate for Exilis, however she said that Ultherapy would be better and longer lasting. I thought my arms had some fat and she said that is was skin. Could someone comment on the difference between the two therapies and if they prefer one over the other.