While both Omega-3 and 6 are essential, maintaining ratios similar to that of natural forage is likely optimum (Kellon, Nutrition as Therapy) and horses on hay diets with little or no access to good pasture should be supplemented with an omega-3 source. Cured forage rapidly loses it omega-3, and many other popular supplements such as rice bran and black oil sunflower seeds (BOSS) are high in omega-6. For example, to "balance" the omega-6 in two ounces of rice bran you would need four to six ounces of flax or four to six Tablespoons of flax oil. It would take six or more ounces of flax to balance the omega-6 in one ounce of BOSS. Flax seed also quickly loses its omega-3 activity after grinding unless refrigerated immediately or milled using a special process to stabilize the fatty acids.
You can estimate the amount of omega-3 and 6 in several different additives using the Excel calculator at http://www.desertequinebalance.com/supplements/omega-3. A PDF version is also included for reference. You're not likely to achieve perfect "natural forage" omega-3 to 6 ratios but this can help you keep a more natural balance and avoid feeding excess omega-6 in additives.
There is also a newly updated chart of products supplying omega-3 fatty acids that you can use to compare daily cost. The obvious front runners in cost were "generic" flax seed and flax oil. Next in line are three "stabilized" products: HorseTech's NutraFlax, Omega Fields' Horseshine and Triple Crown's OmegaMAX. Karron Oil, once a Horse Journal top pick, is currently unavailable in the US. Platinum Performance weighs in with Equine Healthy Weight, flax oil which uses vitamins C and E as preservatives. Chia seed is fairly equivalent to flax seed in omega-3 content. It remains pricey but this may be made up for by the convenience of being able to feed chia seeds whole without grinding.
I'm not a fan of giving any animal-based products to my horses so only one product containing fish oil is included. It has not been conclusively shown that the form of omega-3 in fish oil is well utilized by horses (as it is in humans).
There has been no documented evidence that either flax seed or chia seed can prevent sand build up or clear sand from the horse's digestive tract. Until this has been clearly demonstrated you should follow your veterinarian's suggestions for sand build up prevention.
I hope you find this information useful - it isn't always simple to balance cost vs need or convenience.
Desert Hoofprints in 100+ degree Arizona where flax flowers grow under my bucket washing rack.