Here is a link to a summary of a nice well done study on this question.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3150191/figure/fig3/

What is important to know about supplements with:   

Glucosamine, Chondroitin or Omega Fats (Omega 3 Fatty acids)

1. With or without  Chondroitin.   Remember this; all animal source glucosamine (GA) supplements contain chondroitin sulfate (CS),  whether listed on the label or not.

2. Dose.   Studies comparing the glucosamines from cartilaginous fishes (shark, mussels, clams, shimp, crab) v.s. animals (cow, chicken, pig)   all have the same beneficial effects when the dose of GA & CS is made the same.  The amount of active ingredients in each gram of fish cartilage is much higher than animals. This means less powder needed  if you use fish compared to animal cartilage. This explains why capsules from shark or green mussel  work better than the same size capsule made of cow, chicken or pig cartilage. The same size capsule has more active ingredient when made from cartilaginous fishes.

3. New stuff- a plant based, synthesized form of glucosamine.  A word about the soybean derived formulations of glucosamines. The available information comes from the company that makes the product. They claim that it works as well as other GA products. I read the studies & found a few problems. The product was tested on only a few animals (<12 dogs) for a few weeks and there was no control group. I could not conclude from their information that these products are equal to natural GAs. Publications that directly compare the soy v.s the cartilage based supplements are needed but haven’t been published yet.  Since Cosequin(Soy based) is more expensive than most shark or green mussel nutriceuticals, I don’t recommend it at this time for dogs or cats.

4. Fish oil or Omega fats= Omega 3 fatty acids.

The high quality fish oils contain the omega 3 fatty acids that reduce inflammation. These are from Salmon, Mackerel, Cod Liver oil. Plant oil sources of Omega FAs like: Flax, Borage, Safflower that are converted to Omega3 FA by  people, horses, cows are not metabolized by dogs or cats into a usable form of Omega 3 FA. Many carnivores lack the liver enzymes that covert these fats for use. So read the labels of all supplements to see if the Omega 3 FAs are all from fish sources.

5.One more important point to remember: Omega 3 Fats are not stable when heated .  So dry/packaged treats or dog foods that are “fortified” with Omega 3 fats, and have been cooked, have destroyed the Omega fats, a waste of money. Fortified and other “catch words” are added to product labels  to sell more product. If Omega 3 fats  is not listed on the label in mg amount, chances are it is not high enough. Food companies are happy to list the exact amounts when they are using the good stuff. I find many will mix, <20% fish oil with 80%+ vegetable oils for cooking. Then just list the total omega fats.   Most veggie sources have Omega 4, 6, 9  fatty acids. This example would be 20% Omega 3 and 80% Omega 4 & 6.

6. Omega 3 fats have a short shelf life, should be refrigerated, and never smell strongly fishy.  Strong Fishy odor = fats that are spoiled. Belching of fishy taste/odor indicates the same problem, the product is spoiled. Enough spoiled or “rancid” fat can irritate the stomach.

I  recommend Now® Brand of either Shark or Sea Mussel capsules and Shark powder when available. I have switched to the Veterinary prescription brands for fish oils because these are filtered and concentrated. Which removes the contaminates from the oil, such as pollution and heavy metals. Plus these are concentrated which makes the price of the Omega 3 FA, based on milligrams (mg) the same as the over  the counter brands. If not available in your area look for a cold pressed Atlantic Wild Salmon oil,  hopefully without flavors for dogs and cats.