18650 battery cells are 18 mm in diameter and 65 mm in length. They have a long track record of strong performance in the industry. They are easily available in the market and provide good weight distribution and energy capacity for your scooter.
On the other hand, 21700 battery cells are the new kids on the block. They are 21 mm in diameter and 70 mm in length. 21700 cells don’t have a strong proven track record yet. Their biggest advantage is the increased capacity they provide you compared to 18650 cells. Your scooter will have much faster acceleration and longer mileage if you use them.