Bagels - Baking Soda or Sugar in the Water?
The main purpose of adding things to the boil are to promote browning and flavor. The texture of the crust is more a product of gelatinizing the starches by boiling, using a high oven temp, and using a good amount of steam during baking.  

You can increase browning two main ways, by promoting the Maillard reaction and/or promoting caramelization.  Baking soda, lye, or sodium carbonate baths increase the Maillard reaction, and increase surface gelatinization by breaking down some starch, but also give the bagels a "pretzely" taste.  You can also promote the Maillard reaction by adding protein to the surface by using an egg or milk wash.  Sugar (white or dark), honey, and malt added either to the boil and/or the dough increase surface caramelization and also lend a subtle glaze.  

Personally, I always use malt incorporated both into the dough and into the boil.  It adds a subtle sweetness and cereal flavor that complements the bagels. I find the pretzel taste from an alkaline bath off-putting.  When I don't have malt I use honey, molasses, or brown sugar. I don't use an egg wash either, if you want to add toppings just place the bagel into the topping right out of the boil... they'll stick without anything else.