We consider uplink transmission for a massive multi-user multiple-input multiple-output (MU-MIMO) system in which the base station (BS) is equipped with a mixed analog-to-digital converter (ADC) architecture. In this architecture, a portion of the antennas at the BS is connected to low-power low-resolution ADCs while the other is fed to power-hungry high-resolution ADCs. By taking into account the mixed-ADC architecture, we derive a closed-form expression for the sum spectral efficiency (SE) when maximum ratio combining (MRC) detection is employed at the BS. Then, we formulate an optimization problem to determine, for a given power budget at the BS, what is the optimal distribution of low and high-resolution ADCs that maximizes the sum SE. Interestingly, it is shown that in most realistic scenarios, using a larger number of antennas with only one-bit ADCs is the best approach.