Merit works like this:
If you define a rule without specify to whom, the points will be assigned to current_user by default
score 15, on: 'reviews#create' # This is for current_user
If you want to assign points to an user outside of current_user, specify it
score 10, :on => 'questions#vote', to: :user
In above example, :user
refers to method question.user
, who is the author of the question, different from current_user who voted this.
In OP's case, actually ActiveRecord Reputation and Merit are for different purpose and can't be fully interchanged.
ARP can be used on all models, including users and non-users. Meirt is for user only.
Merit has another module for badge. ARP doesn't.
Merit has a Rule module where you can define all rules in one place, like CanCan. While in ARP there is no such place.
You'll deal with models mainly in ARP. In Merit, you'll work with both Controller and Model.