A follow up to my last post.
The AF assist of the YN is one set pattern and quite bright in comparison to the 600-EX-RT, also being a fixed projection pattern the height of that pattern varies in the viewfinder with subject distance such that at close distances you might find the pattern too high for lower AF points. As the subject distance increases the pattern goes lower in the viewfinder.
The 600-EX-RT has a much broader pattern, or set of lines, depending on what AF point you have selected, the output is also dimmer and less distracting, but obviously bright enough as it works very well.
Using my 50 f1.4 I'd say the 600 AF assist pattern enables faster AF in very dark situations than the YN does, there isn't much to it but along with the variable pattern from the 600 that is tuned to your actual AF point I'd say there is an observable advantage to the Canon. The YN AF assist works very well though.