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.