Polarization Diversity (PD) is a favorable option for compact Base Station (BS) and Mobile Station (MS) antennas while offering the benefits of Space Diversity (SD) for wireless systems. The large bandwidth of future networks is more likely to cause frequency-selective fading yet compactness is desired irrespective of terrain. Therefore, in this paper, performance of PD for flat-fading and frequency-selective fading is investigated based on the Symbol Error Rate (SER) with emphasis on frequency-selective fading. Nakagami-m fading is employed as it can represent a wide range of multipath channels raging from line-of-sight (LoS) to non-line-of-sight (nLoS). SER of PD and SD are compared based on SNR. The dependence of SER of flat as well as frequency-selective fading on SNR, PD factors and fading parameter m is presented. SER of frequency-selective Rayleigh and Rician fading is also compared in this work.