Soy, having a higher protein count has more nitrogen released to the lawn and therefore may be a better fertilizer. My first and last organic applications of the year are cracked corn primarily to fend off fungal diseases. In my area, soy meal costs just a little more than corn but is worth it for the higher nitrogen release and to diversify the food for the soil microbes. Last year I used alfalfa once before overseeding some fine fescue areas with KBG. I couldn't comment of the effectiveness of that however. Last year, alfalfa was more expensive than soy, corn was the least expensive.
Bill Hill

Soy has about double the nitrogen in proteins (soy's a legume, so it grows by using nitrogen-fixing bacteria, hence it uses a lot of nitrogen very freely in its structure).

If the price is competitive to corn meal/cracked corn (no more than double), it's worth it.