Type of Work
MCSE or Microsoft Certified System Engineers are trained in order to build new systems. They design it and then execute the plan as per the design. The system is made from the ground up as per the specifications of the client and many technical aspects have to be kept in mind.
MCSA is a Microsoft Certified System Administrator. The duty of an MCSA is not to build a system but manage it once it has been put in place. Rather than designing and making it, the administrator runs the show afterwards.
Level of Testing
The level of testing for an MCSE is quite a bit hard in comparison with an MCSA. The former has to take seven exams and the latter has four. Since they have to do a tougher job, they are tested at a higher scale as well.
The pay of an MCSE is higher than that of an MCSA. Not only the level of testing is difficult, the actual job itself is quite hard and therefore, the system engineers are able to make more money than their administrative counterparts.
If you are an MCSE, it does not mean that you already are an MCSA. If you are looking to be an MCSA, you will need to take the four exams needed for that. Similarly, if you have done an MCSA, you will not be getting any exemptions in the MCSE and you will need to take all seven exams. Thus, in order to be successful in becoming both at the same time, you will need to take all eleven exams regardless.