The question is self explanatory. Which of the two should be considered when selecting a reaction wheel(which does not function as a momentum wheel)?
Both function as reaction wheel because mechanically it's still the same device. Since what you're looking for in a reaction wheel is torque, the torque control mode offloads the need of torque control from your system onto the reaction wheel controller, meaning less work - but also less control over the device. In speed control mode you must derive the necessary speed changes from torque demands - more work (calculations) for your attitude control system, but then you can account for factors manufacturers of the controller didn't and apply in their algorithms that e.g. take precession into account or anticipate disturbances from other reaction wheels even before the sensors pick them.
Shortly, "torque control" is "easy mode", where you forfeit some of the control flexibility in favor of ease of use. "Speed control" will necessitate somewhat more complex algorithms, but allow for far more advanced algorithms.
(technically you could use these more advanced algorithms with torque control mode but that would add yet another layer of complexity, in revealing the variables that are hidden by this method of control; completely unnecessary).