Coming from a position of personal experience. I had a full scholarship in college for athletics, and now 20 years later, I am feeling the abuse I put my body through. Worked in a physical therapy clinic for 6 years. Also helped rehab my 93-year old father after his double hip replacement.
1. Weight. If you carry any extra, get rid of it. Its like needlessly carrying around a backpack full of rocks. The extra weight wears on our joints.
2. Flexibility. I sit most of the day behind this computer screen. If I don't take the time to stretch my hips/legs, my back kills me, and i'm unable to stand comfortably for any length of time. A body is meant to be in motion.
3. Core Strength and stability. No strength = no stability. Think constructing a high rise on marsh land vs a concrete foundation.
If your arthritis has compromised your hip/knee joints, there's nothing that can fix it save joint replacement, but further degradation can be slowed with proper nutrition, supplements, and exercise.
Both Tai Chi and Yoga are great for core strength and flexibility. Tai Chi contains more of the 'exercise' component.
Do both.
my $.02
Rob