Hi Karthikeyan,
Ben makes a good comparison. To reinforce it, consider how each function is treated: when Promoting a component, the base will automatically be hidden in the assembly for you. This works prefectly when adding assembly level machining.
Wave linking does not automatically hide their sources. This is appropriate when linking data between components but can be a liabilty when linking entire bodies for say assembly level machining. Still, as Azrael points out, Wave links can be more flexible, and so they should not be ruled out if Promotions cause problems.
For more information, see the Assembly Best Practice sections of the on-line help.
Cheers,
-Mike