The competition of electron-electron interband scattering (ee) and longitudinal optical phonon emission (e-ph) as electron capture mechanisms is theoretically investigated in III-nitride quantum wells. The non-trivial separation of their scattering probabilities is discussed, and compact expressions for capture time are obtained in the framework of the quantum many-body formalism. At the typical operating conditions of light emitting diodes (LEDs), the model predicts an increasing importance of ee scattering as a capture mechanism with increasing carrier density. Verifications against recent experiments are presented to support this finding and confirm the need for population-dependent capture time expressions including both ee and e-ph mechanisms for an accurate description of LED carrier dynamics and efficiency. Published by AIP Publishing.