Also depends on the cadence that you're comfortable riding at. I tend to pedal with quite a slow cadence and a high torque so the smaller sprockets work well for me. There have been times, on very steep downhill runs, where I've ran out of gears - sat on the big ring at the front and the small one (11T) at the back and just not been able to pedal fast enough! I would have been quite happy to have had a smaller gear on the back at that point