I have a 21d, it was cheaper than the 7D at the time and is still cheaper. The 27d I suspect is cheaper because more people prefer the single derailer on the rear. The market has decided that a 9 speed is more in demand. When I rode the 7d it felt like a good all around bike. The 21d had more range of gears that I could use, but was more complex to operate. I loved both bikes.