Comparison between scheduleUpdate and scheduleUpdateWithPriority in cocos2d

In cocosd, CCLayer provides the schedule method to refresh frames. The default frequency is 60 frames per second, as shown below:

Init {
[Self schedule: @ selector (tick :)];
}
-(Void) tick :( ccTime) dt
{
// Update your stuff here
}

Cocos2d v0.99. also provides the following two more effective methods to refresh frames.
[Self scheduleUpdate]; // use priority = 0 ---
[Self scheduleUpdateWithPriority: an_integer]; ---- B
Then implement the update method in the cclayer subclass.
-(Void) update :( ccTime) deltaTime
{
// Update your stuff here
}

What are the differences between the old and new methods?
The new method provides priority options, which means you can sort the execution sequence for the update method of n brother.
The new method can save some memory and increase the speed of each update.