M.P.
I feel that Pilates is more to tone and to get into shape and yoga is more for stretching and stress relief. They are both good to do. I do 10-15 minutes of yoga in the morning to stretch out and then pilates seperately. If you can take some classes you should-both are very specific in their form and with your bad back you should have an instructor make sure you are doing it right so you don't hurt yourself. Yoga might actually be the better place to start until your back gets better.