In some ways, this question is impossible to answer.  You have identified that you can't keep both tip speed and torque per volume constant.  However, the difference in flow patterns between five impellers and three impellers could still be a factor, even if you could keep torque per volume and tip speed equal in both sizes.
 
In the simplest analysis, you need to decide whether equal tip speed and its possible effect on dispersion is more important than equal torque per volume and its likely effect on fluid velocity.  Assuming that liquid motion is the more important factor and the impeller types are the same in both scales, equal torque per volume is probably the best approach.  However, if a complicated chemical reaction is a factor, equal power per volume may be a better scaling approach.
 
Your problem is not just deciding which scaling approach is better, you need to decide what factor is controlling your mixing application.

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