Typeracer is generally more accurate, but there are pretty stupid passages with an overabundance of numbers. That, and the sometimes ridiculously inflated high scores (Jack getting >200 wpm, for instance, on a very short passage) which are completely inaccurate. While I feel 10FF is slightly inflated, it does help you type words very quickly, which is important for increasing speed overall, and it doesn't have a particularly wide range of results.