this is my concern as well. when i do an ethanol extraction i get a 10% yield. however, the rosin press gives a 25%+ yield - i am assuming the additional yield is plant wax, from trichome bodies, etc.
so i stick to flower.
The entire plant is coated in epicuticular wax. When you vape flower you're heating all that wax up regardless.
As far as yield... Cannabinoids are more soluble in ethanol at ambient temperature, but many people use cold ISO for the QWISO process to minimize pulling chlorophyll and other polar compounds. This affects the contact time and can easily reduce yield. A catch-22 of Etoh extraction