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Electronic predistortion (EPD) to compensate for chromatic dispersion is an attractive option to replace inline optical dispersion compensation (ODC). In this paper, we present a study on the impact of intra- and interchannel nonlinearities in EPD systems compared to optimized ODC systems at 10 and 40 Gbit/s for NRZ-OOK modulation. First, the theoretically achievable nonlinear tolerance of EPD and ODC systems is studied by neglecting the EPD transmitter's hardware limitations and the dispersion compensating fiber's loss and nonlinearity. At 10 Gbit/s, EPD shows stronger degradations due to intra- and interchannel nonlinearities than optimized ODC. We extend existing studies for 10 Gbit/s EPD by analyzing the relevant launch power levels before interchannel nonlinearities limit the performance. The limit is 8 dB larger for ODC than EPD at 10 Gbit/s. In contrast, operating at a bit rate of 40 Gbit/s significantly reduces this difference in the nonlinearity tolerance between EPD and ODC both for single channel and WDM transmission. The maximum power per channel of 40 Gbit/s EPD is only 1 dB smaller compared to ODC. We then conduct a more realistic comparison at 40 Gbit/s by including the effects of 60 GSa/s digital-to-analog conversion with 4-bit quantization in the EPD transmitter and by considering the loss and nonlinearity of the dispersion compensating fiber. Analyzing the optical signal-to-noise ratio margins confirms that the performance of the realistic EPD system is similar to optimized ODC making EPD more attractive for electronic dispersion compensation at bit rates of 40 Gbit/s and above.