A controversial technical discussion is raised whether WiMAX or LTE is the most promising 4G technology. Since LTE and WIMAX as 4G systems have both adopted OFDM as the preferred physical technology for better spectrum efficiency, the RF parts of those systems have to face major challenges. Hence the extensively applied error vector magnitude (EVM) can be used as an effective and efficient method to evaluate the RF performance of those digitally modulated communication systems. Considering an optimal subcarrier allocation in OFDM-based systems Spectrum Flatness is an important quality characteristic, which has been rarely examined in the past. In comparison, effects on EVM, related simulations and measurements have been sufficiently analyzed in the past. But particularly the correlation between EVM and Spectrum Flatness has not been taken into account so far. In this paper we are discussing EVM measurement accuracy and furthermore the influence of sliding window algorithms on the EVM since those are used in LTE systems. On the other hand we will show that only using sliding window algorithms does not make spectrum flatness measurements, as standardized in WiMAX, dispansable in order to promote the decision process. For this purpose an accurate, robust EVM measurement algorithm, simulation, application and results are presented.