I don't know much about Pilates, but yoga would help. Two weekly classes would be great; I think the key is for you to find "a good class" so you enjoy it, then it will feel like less of a commitment!
Yoga has physical benefits (flexibility, strength, stamina) but also mental benefits (concentration, calm, confidence). Unless you are going to go for it with a really strong, powerful practice every day then it won't help you tone up; but you should find over time greater flexibility, and it should help you to switch off from the stressful outside world for at least the period of the class.
I would suggest a class to start with, at least, rather than purely practising from a book or DVD (I am a teacher so I would
) because then you have a teacher looking at you, who can correct you and help you to progress. A weekly class, supplemented with practice at home, would be a good starting point - but as I say, you need to find a class and teacher you like otherwise it will feel like a chore.
I practise Iyengar yoga, so of course would recommend that. A good teacher will ensure they adapt the pose to your level of stiffness and help you to work in the way that will alleviate it.