I really don't know much about Yoga, and less about Quigong. So I can't make a recommendation based on experience or specific knowledge.
However, Asian exercise and energy systems are indeed very valuable in terms of antiaging, because they take into account many aspects of health and wellness, not just burning calories or building muscle.
I don't think they are a substitute for strength training, but rather go hand in hand with it, filling the gaps that strength training leaves with the other things you need like flexibility and enhancing energy in your body.
They are excellent for stress as well, because they help with proper breathing, and have a meditative aspect to them that helps calm and center you.
So to sum up my feelings about Yoga and Quigong, they don't replace other forms of exercise, but rather they are a very valuable addition to your antiaging regimen.
