Hi Tushar,

or both of them the Order and Criteria (during the creation of Escalation and Assignment Rules) are same , so it doesn't matter much .
 
Coming to the differences between them .
 
Case Assignement Rule : You can specify whether you want this to be the active rule for  cases created manually and via the web and email.
 
Case Escalation Rule : You can just specify whether you want this to be the active escalation rule .
 
Case Assignement Rule :  You cannot Specify business hours criteria
 
Case Escalation Rule : You can Specify business hours criteria
 
Also for a Case Escalation Rule you can determine what field applies to your Age Over number of hours. Your Age Over setting can be based on the number of hours since:
 
A case was created
 
The case was created unless it has been modified; once modified, the case will never get escalated
 
The most recent time a case was modified
 
For example, if you choose Based on last modification time of the case and your Age Oversetting is 5, cases will get escalated 5 hours after the most recent last modified time and date as long as the case is open.
 
Also the order of execution for Salesforce matters here .
 
Salesforce processes rules in the following order:
 
Validation rules
 
Assignment rules
 
Auto-response rules
 
Workflow rules (with immediate actions)
 
Escalation rules . 

If this helps, Please mark it as best answer.

Thanks!!