Don't try to optimize with these, it's useless. Instead, deactivate allow_short_tags (because of problems when loading XML files) and write clean, readable and understandable code.
Even if there may be a slight difference (which is definitely lower than 10%), it's useles to optimize with it. If your scripts are slow, look at your loops first. Most of the time you can win a lot more performance by optimizing the programms flow than by using strange syntax.