Oral supplements don't help SOME horses. Usually horses that need more help, I.e. Horses with moderate to severe arthritis. That DOES NOT under any circumstances mean it doesn't help horses in good health with mild arthritis or horses just need some support (as in the OP's horse). There is no point in counting out oral supplements when you don't have a problem yet. They are for support, and they work.
I doubt you have ever used Corta-Flx so please don't pass judgement on a product that you have never tried for yourself.
To the OP- tell us what you end up with. I'm curious to see what brand you choose!