Although a Yoga teacher (and Pilates is based a lot on Yoga exercises) I have studied Tai Chi for 9 years and that would suit you as it's good for balance and co-ordination and leg strength. Pilates will focus on your core muscles and a large part of class generally is done from a lying down position.
All classes vary. A good Pilates class will only have a relatively small number of people in it for individual attention.
Tai Chi is, as society described, a moving meditation.
Try both!