Should I use class method or instance method, and why?

In my Rails app, when creating a business I have a form that has the following field:

   <%= check_box_tag(:default_company) %> 
   <%= label_tag(:default_company, "Set Company as Default") %>

Essentially when I create a business, if they check this box, I need it to run something like the following code:

def set_default_company(company, user)
  exists = DefaultCompany.find(user.id)
  if exists
    exists.update_attributes(company: company)
  else  
    DefaultCompany.create(company: company, user: user)
  end
end

While learning, I would usually do that stuff in my controller but i'm trying to follow best practices and use a fat model, skinny controller, so I'm wanting to use logic like this:

def create
  @company = Company.new(params[:company])

  if @company.save
    if params[:default_company]
      Company.set_default_company(@company.id, current_user.id,)
    end
    flash[:notice] = "Company was successfully created."
    redirect_to @company
  else
    redirect_to new_company_path
  end
end

Here is where I am getting confused on whether to use a class method or an instance method, to call set_default_company. They both seem like they would work and I can't see a benefit to one or the other.

In addition to giving me any information as to which method to use, if someone could show me a brief implementation of writing that as a class method vs. instance method it may give me a better understanding as to why.

Here is how I would write them:

def self.set_default_company(company, user)
  # Logic here
end

def set_default_company(company, user)
  # Logic here
end

Writing them that way I don't see a benefit to either.