Based on my experience with T'ai Chi Chu'an classes years ago - which were complemented by warming-up sessions using Qigong - both provide an excellent means of meditation. Qigong, while it involves stances and some repetitive movements, is in terms of meditation the most like seated meditation styles. It stills the mind, steering it to focus on just one unchanging pattern.
T'ai Chi, on the other hand, involves a more complex set of movements. The endeavor to learn those movements in sequence, to initially perform those movements in near-perfect compliance to the instructions of a teacher, and ultimately to feel the energy of those movements and let them gently guide your body into the pattern best suited for it, requires unwavering concentration and focus, ultimately putting you into a meditative state which in my opinion puts you into a deep and dynamic relationship with the Force - in contrast to the more passive relationship inherent in Qigong and many forms of seated meditation.
In terms of practical, outer-world benefits - Qigong places a stronger emphasis on physical healing, while T'ai Chi offers a method of self defense.
If I were taking lessons now, I'd opt to T'ai Chi. I really like the sense of unity with my surroundings that it provides. If you're still trying to figure out which discipline to begin with, you can't go wrong with either choice; but I'd recommend planning sooner or later to try both, so you can discover which suits you best. The answer won't be the same for everybody.
