  We present a study of two spectral lines, Fe I 6173 Angstroms and Ni I 6768
Angstroms, that were candidates to be used in the Helioseismic and Magnetic
Imager (HMI) for observing Doppler velocity and the vector magnetic field. The
line profiles were studied using the Mt. Wilson Observatory, the Advanced
Stokes Polarimeter and the Kitt Peak McMath telescope and one meter Fourier
transform spectrometer atlas. Both Fe I and Ni I profiles have clean continua
and no blends that threaten instrument performance. The Fe I line is 2% deeper,
15% narrower and has a 6% smaller equivalent width than the Ni I line. The
potential of each spectral line to recover pre-assigned solar conditions is
tested using a least-squares minimization technique to fit Milne-Eddington
models to tens of thousands of line profiles that have been sampled at five
spectral positions across the line. Overall, the Fe I line has a better
performance than the Ni I line for vector magnetic field retrieval. We selected
the Fe I spectral line for use in HMI due to its better performance for
magnetic diagnostics while not sacrificing velocity information.
