I've done a few different styles of Tai Chi (Yang, Chen, Wu...). All of the styles have pretty much the same focus of relaxed intent and would be good for your knees to build strength. It is a martial art and quite effective when taught correctly. Some instructors are more martial in their teaching which adds depth to the training. Many non Chinese instructors don't know the martial applications (tofu chi is best to avoid).
I found the key to keeping your knees intact in any martial art, especially Tai Chi is to keep the knee aligned with the foot and not to extend your knee past your toes. If you get out of alignment and twist your foot or knee, you stand a good chance of tearing up your knee. Extending your knee past your toes, even if aligned will put a huge amount of stress on your knee. Don't let your knee collapse in or out or twist when doing the form.
Sink your hips, tuck in your butt somewhat, but don't add tension.
Until you build up the muscles used in Tai Chi, you will introduce some tightness or tension. Just stay relaxed and it will come over time. Keep your shoulders and elbows down or the tension will spread. My teacher always said relax and go lower and slower.
It is probably best to avoid Bagua until your knees get stronger, it is inherently twisting. Hsing-I is more linear, but requires more strength. I'd recommend Yang to start, and the 24 is a good entry point.
That's my .02 cents after years of many martial arts.
