Referencing ScriptableObjects by List index or setting the SO as a variable

Right now I have a GameManager class which holds all the Ingredients, Quests, Recipes, Skills, etc for my game.

Code (csharp):

1.  
2. public class GameManager{
3.     public static GameManager instance;
4.     protected GameManager(){ }
5.     private void Awake(){
6.         instance = this;
7.     }
8.     [SerializeField] private List<Recipe> _recipes = new List<Recipe>();
9.     [SerializeField] private List<Ingredient> _ingredients = new List<Ingredient>();
10.     public Recipe Recipes => _recipes;
11.     public Ingredient Ingredients => _ingredients;
12. }
13.  

With the lists filled by dragging ScriptableObjects to them.
For the Recipe ScriptableObject would it be better to have the Ingredients and the Result referenced as:

Code (csharp):

1. [SerializeField] private List<Resource> _ingredients;

or

Code (csharp):

1. [SerializeField] private List<int> _ingredients;

Does one use less memory than the other? Is either faster?
I already made use of the

Code (csharp):

1. List<int>

to reference the quests already completed by the player character and it works. I'm just not sure if that's the best option or if I should re-do it.

Appreciate your help.