I'd get something with glucosamine chondroitin and msm, I have my old gelding on smart flex senior and my OTTB mare on smart flex maintenance, I don't ride my old guy anymore, and he's out 24/7 so he doesn't normally get stiff, so I can't saw if it helps, but at his age I feel better giving it to him, especially in the winter, my mare never showed any stiffness, but with her being off the track I wanted to put her on it at a preventative since I know they OTTB can be more prone to joint problems form being worked so hard so young...anyway I found this chart really helpful because you can see how much of what is in each supplement, and the price per dose
Joint Supplements at a Glance it's hard to pick a supplement when there are so many to choose from