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I couldn't find anything in Google and I don't have any experience with benchmarking of the two.
I can't see any reason why one would be faster than the other though. They are reasonably simple functions and I don't think there is any validation etc. that would slow either down. I think it just comes down to preference.
PHP includes can be a little more powerful though as you able to state mandatory pages for include using require(). You can also include pages off other domains which you can't do via SSI.