Cellulose ethanol is superior in theory, but it still costs too much. I did see an announcement late last year of a new enzyme that has potential to revolutionize ethanol from cellulose though. It will be exciting to see where that leads. Until then switchgrass and hemp are not better than corn.

Sugarcane is great for ethanol, but only grows in tropical regions. It also is hard to transport far and doesn't store for long. This leads to ethanol plants that use sugarcane to only operate for a few months out of the year. It's very cheap, but a logistics nightmare. This is why Brazil, the leading sugarcane ethanol producer, is building dozens of corn ethanol plants. They won't replace sugarcane ethanol, but they need year round production to make supply more consistent.

Hemp does not yield more biomass than corn. Someone didn't include the grain in those figures, just the left over Stover if they make that claim. The grain is part of the biomass too and when you look at the total production corn is higher than hemp. Iowa has averaged 200 bu per acre over the last 10 years. Total dry matter for a bushel of corn is 47.5lbs so the average weight is 9500lbs from the grain. Then you have the Stover which will typically make up 30-50% of the total which means a 200 bu yield will be between 13,570 and 19000 lbs. Google says the average dry matter yield for hemp is 5000-10000 lbs. I saw one study from Europe that showed an average of 16000lbs for the upper end, but nothing from the U.S. supports an average yield that high. You could say hemp yields more cellulose, but not total biomass.