This also comes down to personal preference. Tuna packed in water is lower in calories and fat compared to tuna in oil. If you're looking for a lighter and lower-calorie option, water-packed tuna is a better choice. This version also has a milder flavor so it doesn't take away from the other ingredients.  Tuna packed in oil on the other hand, has a richer flavor and a more indulgent, flakier texture. The oil adds a layer of richness to the tuna that can be appealing in a salad. The oil can serve as a natural dressing almost eliminating the need for as much mayonnaise.