We have used a combination of high-n Rydberg spectroscopy and zero electron kinetic energy (ZEKE) spectroscopy to measure the adiabatic and vertical ionization energies of 1,4-diazabicyclo[2,2,2]-octane to high precision. A comparison of the two methods revealed that where extended and unperturbed Rydberg series can be observed, the precision with which the ionization energy of a molecule can be measured is up to six times better via Rydberg extrapolation than with high resolution ZEKE spectroscopy. The difference in precision derives, not from any inherent shortcoming in the resolution achievable with ZEKE spectroscopy, but from errors that arise in evaluating the field ionization redshift. Accurate vertical ionization energies of 59 049.0 +/-0.1 and 58 889.9 +/-0.1 cm(-1) were obtained for ionization to the 24(+1) and 25(+1) vibrational levels in the ion. A value for the adiabatic ionization energy of 58 033.3 +/-0.3 cm(-1) has been measured by ZEKE spectroscopy for the first time. The precision with which we have been able to account for the field ionization redshift has been improved by calibrating the ZEKE spectrum against the nu (25) Rydberg spectrum. The ZEKE spectra presented here were recorded using a double inverted pulse sequence with field strengths as low as 140 mV cm(-1). The result was exceptionally well- resolved spectra revealing the rotational contour of each ZEKE band. For the 24(+1) and 25(+1) bands the spectra revealed a strong central Q branch, with weaker P and R branches, consistent with a propensity for angular momentum transfer to the Rydberg electron rather than within the core. In contrast to what is commonly observed in ZEKE spectroscopy, where rotational autoionization often results in branches associated with negative DeltaJ appearing with enhanced intensity, the R branch appears with significantly enhanced intensity compared to the very weak P branch.