As for speed, the only right answer is it shouldn't be your concern at all. Period.
The difference, if you even find one, doesn't matter at all. To be concerned of speed, one should understand whole picture, not one pixel of it. Making your application faster and more efficient is a great and complicated task. But it cannot be done by asking "which is faster" questions.
If you really concerned in that, start from learning what profiling is.
As for advantages, a manual page explains is the best place to learn it: http://php.net/types.string
It's not really advantages but rather use cases though.
