A or B, with reasons

a.rb

# A. Less nested contexts, clearer object collaboration at the cost of repetition

describe PlaceParameters do

  describe '#sort_by' do

    it 'concatenates the sort_by and sort_dir params' do
      params = { sort_by: 'name', sort_dir: 'asc' }
      sort_by = PlaceParameters.new(params).sort_by
      expect(sort_by).to match /^name asc$/i
    end

    it 'explicitly defaults sort direction to ASC' do
      params = { sort_by: 'name' }
      sort_by = PlaceParameters.new(params).sort_by
      expect(sort_by).to match /^name asc$/i
    end

  end

end

b.rb

# B. Contexts used to DRY assertions, object collaboration less obvious at a glance

describe PlaceParameters do

  let(:place_parameters) { PlaceParameters.new(params) }

  describe '#sort_by' do

    context 'when sort_by and sort_dir are specified' do
      let(:params) { {sort_by: 'name', sort_dir: 'asc'} }

      it 'concatenates them' do
        expect(place_parameters.sort_by).to match /^name asc$/i
      end
    end

    context 'when sort_dir is not specified' do
      let(:params) { {sort_by: 'name'} }

      it 'explicitly defaults sort direction to ASC' do
        expect(place_parameters.sort_by).to match /^name asc$/i
      end
    end

  end

end

@gabrielrotbart suggested this.

It still uses the contexts to set up each example differently, but repeats the interaction between the PlaceParameter and the params object for clarity - mirroring implementation code:

describe PlaceParameters do

  describe '#sort_by' do

    context 'when sort_by and sort_dir are specified' do
      let(:params) { {sort_by: 'name', sort_dir: 'asc'} }

      it 'concatenates them' do
        sort_by = PlaceParameters.new(params).sort_by
        expect(sort_by).to match /^name asc$/i
      end
    end

    context 'when sort_dir is not specified' do
      let(:params) { {sort_by: 'name'} }

      it 'explicitly defaults sort direction to ASC' do
        sort_by = PlaceParameters.new(params).sort_by
        expect(sort_by).to match /^name asc$/i
      end
    end

  end

end