The difference of Completing contract or breaking contract depends on the conditions in your contract.
That means if you breake the contract within the contract period you will not be eligible for leave salary, end of service benifit,or Air Ticket depends on your contract. If you complete 2 year contract you will be eligible for the above beifits.

In any case you should have a releas or NOC to work with another company. The difference between release letter and NOC is , with release you can transfer from present company to another company without exit. You need NOC to come back to Qatar with a new visa after exit and to avoid two year ban

Abdul Gafoor