Liz,
Congradulations at this, the start of your journey!
Neither yoga nor pilates will be too much for you. If the tape or instructor you start with is presenting material that's too difficult for you, just put it on the shelf (or on vacation), find one more suited to where you're at right now, and begin there. It's always the trying to 'jump in at the deep end' that sabotaged all my attempts at an active lifestyle. My love affair with yoga began and continues now almost 7 years because we took things SLOWLY!

The primary difference between yoga and Pilates is that Pilates is a brilliant set of physical exersises that, when performed correctly and dillegently, can produce astounding affects on the physical body.
Yoga, on the other hand, is an ancient method of self-care that attempts to bring the body, mind and spirit into harmony and in doing so improving both the physical, mental/emotional and spiritual aspects of our lives.
Namaste,
BrianClt