Jan Ozer from StreamingMedia.com has put up an article comparing the two codecs using Sorenson Media encoding tool. He concludes that the differences are negligible although I think h.264 should be better. He listed many clips for comparison.
Quavlive also gave another comparison:
no conclusion was put. However from these clips, h.264 (x264) certainly is doing better in picture clarity/sharpness and color.