Summarize the following:
They will all deliver the omega 3 fatty acids so form is  a matter of preference. The oil tends to be more “fishy” and the powders often contain other vitamins or minerals. Depending on the dose, capsules can be large, but most are enteric coated. Enteric coated means the capsule will not dissolve until it reaches the intestine. This helps alleviate upset stomach or having a “fishy” taste in your mouth. Some supplements will have more than just omega 3 fatty acids. They may also have vitamins, minerals, herbs or other nutrients. Always check the label to make sure you know exactly what is in the supplement to avoid taking anything you are unsure of. The FDA regulates supplements, but not as strictly as drugs. It is important to check the supplement to ensure you can trust taking it. You want to avoid anything that might be high in mercury or other environmental contaminants. Most companies will put information on purity testing on their label. If you don’t find any information on this, you should consider another supplement.

Summary:
Choose the best form for the supplement. Be wary of other ingredients. Check for purity testing.