Sardines have low mercury content and high selenium, and they usually come canned in water or oil.
One might prefer those canned in olive oil since it’s high in MUFAs, though it also has more omega 6 than 3. People may want to eat sardines to increase omega 3:6 ratio. Also, is it possible for the olive oil to “remove” omega 3s from the sardines, forcing people to consume the olive oil?
In water, it has much more omega 3 than 6.
What do you mean, the small ones or the large sardines? The small ones have more surface area to be more susceptible to changes in the diluent. But many don’t like to eat them as they are canned whole.
I heard a while back somewhere here on reddit about how water is an acidic solvent, and can leech out potential harmful elements in the sardine can, but oil is not, and can actually better preserve the fish without potential leeching.
I would say it primarily makes a difference in the calorie count. If you’re eating these for the protein as part of a weight loss plan, then you might choose the water ones. If you don’t care about that, then go with whatever you think tastes the best.