Testing Rails model validations with RSpec, without testing AR itself

Lets as setup we have model User:

class User < ActiveRecord::Base
  validate :name, presence: true, uniqueness: { case_sensitive: false }, on: :create
  validate :password, presence: true, format: { with: /\A[a-zA-z]*\z/ }
end

A see several ways to test this:

it { expect(user).to validate_presence_of(:name).on(:create) } 

or

it do
  user = User.create(name: '')

  expect(user.errors[:name]).to be_present
end

My main question is which of the approaches is better and why? Can suggest me different approach?

Additional questions:

• How much should I test? As an example, I can write so many tests for the regex, but it will be hell for maintenance.
• How much you think will be full test coverage in this example?