I have done cob and soil cement floors, Paul. The cob is by far a better floor.
Clay about 30%, Sand including all aggregates in your clay if natural clay, 70% (at my place that works out to about 50/50) Straw so a good amount is in every handful.
Trowel it smooth. Let it shrink and crack as it likes. Fill the cracks with a well sanded clay sand grout to prevent shrinkage. Put about 4 coats of boiled linseed oil and let dry. Keep brushes and paper clean up materials disposed of safely as they can spontaneously combust if in a pile creating a fire.
After the linseed oil dried we put an acrylic sealer on top of it. A durable soft floor with a feel and texture/softness like linoleum.
I had trouble finding a good pix of it but here is a bit of it in the foreground of the cob conversation pit area. The step is cob also. It's been there for about 8 years and the only damage is a tree root growing under it cracking it a bit.