To make a third factor a bit more explicit: there's a limit to the power you can apply to a wheel before it slips on the rail. Having more powered wheelsets me

To make a third factor a bit more explicit: there's a limit to the power you can apply to a wheel before it slips on the rail. Having more powered wheelsets means the total possible power is increased. Commuter trains usually use multiple units, and metro trains always do -- they start and stop often, so the improved acceleration is a benefit. Long distance trains can be multiple units, or a single locomotive: the time saving is less useful on a train that stops infrequently, so lower capital + maintenance costs win (plus passenger comfort, it's quieter inside).