Availability

Atlas: 4-24 kg (9-53 lbs)
Apollo:8-34 kg (18-75 lbs)

Size Comparison

Atlas: It's roughly 20% smaller than Apollo in its body (the handle sizes are about the same, and the window - the space inside the handle - is actually a little larger in Atlas, making it a great handle for two-hand swings). Atlas' smaller size makes it easier to swing two bells between your legs and keeps the weights in better alignment with hip and shoulder joints for smaller framed people.

Apollo: Comparable to traditional competition kettlebells, its larger size is better suited for people roughly 5' 8" and taller. The larger size also means the contoured arm rests provide more surface area, making them even more comfortable in the rack position than Atlas - so a smaller person who is training long sets of jerk or long cycle may prefer Apollo for that reason.

Performance

Both Atlas and Apollo kettlebells have great balance and maneuverability, but Atlas moves more quickly. If you like to sprint, juggle, or are used to more compact hardstyle kettlebells, you may prefer Atlas regardless of your size. If maximum comfort, heavy weights, or high repetition sport-type training are priorities, go with Apollo.