Dir Lightmap Performance VS Single Dir Light

From what I understand, you get the first scene Directional Light (no shadows) free of charge (performance wise).

If I have a scene with pixel shaders (normal / spec etc) then which would be cheaper?

1. A directional light in the scene

2. No lights, but using a Directional Lightmap.

I would think that method 1 would be cheapest? Is there a big difference between either method in performance terms?

Note: This is for a PC web game.