I think a simpler solution might be to select the source in for. Additionally you could use || in this case. But otherwise if(@formatter_things != nil, do: @formatted_things, else: @unformatted_things) also works as an expression.
<%= for thing <- @formatted_things || @unformatted_things do %>
<%= thing.name %>
<%= thing.age %>
<%= thing.whatever %>
<% end %>
