I bought the lovely Mrs_Bob a Cub Cadet because she said the colors of the Cub coordinated well with her summer outfits. What colors does your mom wear?
All 3 are great tractors. As previously stated, I would also note that the CK20 is not a Sub-CUT, is is a small frame CUT, it will easily out tractor the other two machines and offers more ground clearance, higher seat height, taller overall stature. Typically a new Sub-CUT is going to offer fairly limited ground clearance, be available in HST only. To my knowledge, the following tractors are Sub-CUTS: NH TZ series, Case/Farmall DX"e" series, Kubota BX series, Massey GC series and JD 2210.
Other small frame CUTS that would be of similar stature, ground clearance to the Kioti CK20 would include JD 4010, NH TC21, Case/Farmall DX21, Mahindra 2210(?), Kubota
B7510 and several others. The only downside I see with these tractors is the fact that they are taller than the SUB-CUTs and their can put your mom's eyes at a level were low branches from small trees may exist. For mowing, you might want to stick with the lower profile tractors?