As an "early adopter" of PHP6, Michael Bommarito has worked up some benchmarks he performed comparing PHP 5.1.3RC2-dev against the latest (at the time of writing) PHP 6.0.0-dev snapshots.
I've carried this PHP6 benchmark series on with Drupal, phpBB, and MediaWiki. The results seem to agree that performance is near-identical, and, given applications written for PHP6's native Unicode, would likely lean in favor of PHP6.
He describes the specs of the system he used ot test it and links to the software he used. His tests (56 of them) ran, measuring response times of the functionality in seconds. The end result was a bit surprising - PHP 5.1.3RC3 came out on top, but not by too much. He's not surprised by the results, however, noting that a lot of the slowness in PHP6 is being cuase by its string handling methods. PHP6 is slowly gaining on the 5.1.x series, and:
My overall instinct is that if a benchmark were written to fairly compare the two (i.e. mbstring vs native Unicode, instanceof over is_X), the results would be even more favorable for PHP6.