It really depends on the type of video you are streaming and the quality you are looking for. Generally, x265 is better than x264 as it offers better compression and quality at the same bitrate. However, x264 is more widely supported and will usually provide better streaming performance over cellular. If you are looking for the best balance between quality and size, you should try encoding with both codecs and compare the results. For example, you could encode a blu-ray at 4K resolution with a bitrate of 20Mbps in both x264 and x265 and then compare the file sizes and quality. This will help you decide which codec is best for you.