Are you stuck on feeding a supplement? I've always felt that feedthroughs just make for really expensive urine...if it's overfed, that's exactly where it goes.
The best things I have found for joints are:
-MSM - it is very inexpensive (just buy the 99% pure...not mixed with anything else...literally pennies a day), is readily absorbed and utilized by the body, and WORKS! It is an anti-inflammatory. If your horse has joint issues, I'd definitely recommend MSM.
-Glucosamine injections (weekly for a month, then monthly). A bottle of 10 shots (10cc ea) is about $90.00. You can put it right in the muscle on the neck. It's just generic glucosamine...Adequan is great, but only lasts for 72 or 96 hours (can't remember which). There is also a product used "off label" prescribed by your vet called Ichon. It works really well too, and is more potent than regular glucosamine, but not as pricey as Adequan.
- Hyluronic Acid - while there is no scientific "proof" that feed through HA works, Legend HA injections are something many horses receive once or twice a year with fantastic results. It is pricy (and is a procedure that must be performed by the vet), but when you average out what you're spending on cookies, it's not terribly more :)
- Previcox/Equioxx - this is firocoxib, and is used in our area for older horses with mobility issues. Equioxx is the equine version, and is found in a paste at around $11/day. Previcox has the same active ingredient, but costs around $.75 per day. Ask your vet if this option is right for your horse.
What it boils down to is what your horse needs vs. what you are able to provide for him. I'd start with the MSM and see if you can't swing the glucosamine injections. I'm just not a big fan of feedthrough glucosamine/HA, as I feel it gets peed out by the next day :) Talk to your vet - there is (at least in our area) no charge for a phone call, or just ask them what their preference for joint management is the next time they are at your barn.