The gears. You can compare the gears here.
Brownie 3 is the most low maintenance option. The gears are sealed inside the wheel and rarely need cleaning or adjusting. It has one thumb shifter and you can change gear when you're stationary (handy at traffic lights). The three gears are widely spaced out and suitable for most leisure riding.
Brownie 7 has external gears. The chain moves up and down the cogs on the back when you change gear using the thumb shifter. It takes only a small amount of maintenance to keep the shifting smooth. You have seven gears to choose from and the range is suitable for most leisure riding.
Brownie 21 is the most complex option and needs regular cleaning and maintenance to keep the gear-changing smooth. There are two shifters and two sets of cogs (three on the front, seven on the back). Combined, this gives twenty one gears, although you'll likely use no more than twelve of them regularly. The hardest gear will get you motoring along at speed and the easiest gears will get you up an alpine pass (if you feel the urge!).
NB. The Brownie 21 also comes with a carrier rack. This can be purchased separately for the Brownie 3 and 7.