Délices au Beurre (Lye Bread)

When you want to make Brezel or swiss Délices, you usually use lye dough. The technique is to coat the dough just before backing with either:

1. Sodium hydroxide NaOH (2 %)
2. Sodium bicarbonate NaHCO3

Both recipes looks quite similar. The second is usually easier and less risky, but the coloring is not as good that with caustic soda.

What is the chemical reaction of both solutions, are they equivalent? Why professionals prefer using NaOH?