I'm going with red clover and alfalfa in my orchard. The legumes will provide all the nitrogen my trees will ever need, and won't die off during a dry summer so they keep feeding the soil biology. I'm also going to be planting other beneficial plants around my trees, ones that bring up nutrients, or attract beneficial insects.
The last thing i would want is plain grass as the root mass it creates sucks up too much water. Although I will be using winter rye for nutrient cycling.