These are twins with different torque numbers. Same new-generation Moza motor, same refinement that turned the R-range into a proper flagship line, and near-identical scores across our rubric - the R25 nudges ahead only on the overall average and compatibility. What you are really choosing is 21 Nm versus 25, and $200. Twenty-one is already more than most drivers use in anger, so the R21 Ultra covers the majority comfortably.
Step up to the R25 Ultra if you want the true-torque flagship and the headroom, or you are chasing the top of Moza's range and the extra $200 does not sting - it remains the value pick at that level, undercutting rivals badly on price per Newton-metre. For most people, though, the R21 Ultra is the smarter money: the same motor and refinement for less, with plenty of torque. Neither is a wrong answer inside the same Ultra family.