Having spent hours comparing both activities through practical tests, I can, without hesitation, state that images drawn in the dirt have far greater resolution and dynamic range than any work that can be created with spit bubbles. The dirt has much greater depth, that, regardless of any DxO marks, is clearly visible to the trained eye. On the surface, dirt is typically more dry and imbued with a light brown character. By drawing with a stick of appreciable tensile strength, once can reach lower layers of the dirt which have greater moisture... which provides darker tones, allowing much more contrast and tonal range than can be achieved with spit bubbles.
Additionally, if one is most selective about the location of the dirt, a very fine grain of soil can be found. Of course, with proper technique, spit bubbles can be made of very fine size, but they will never surpass the resolution of the finest dirt.
Now, spit bubbles do arguably allow for a more efficient workflow and the artist can often create much more work in a day. So, in the end I suppose it all comes down to personal preference and the needs of the client.
I urge you to pursue your own real world tests before subscribing to internet dribble and possibly making any foregone conclusions.
IMHO,
CB