Make sure the contractor explains the terms of your warranty and that you understand it. Many manufacturers also offer extended warranties, covering parts and labor which you may be wise to investigate. Get a written estimate. To make a fair comparison, be sure the estimates you are looking at are for the services you've requested and are based on the same size and efficiency equipment and the same quality of workmanship. Ask about maintenance contracts.
Many contractors offer maintenance contracts that call for periodic maintenance of equipment.
The fee for such contracts is usually well worth it in terms of obtaining optimum performance from your system. These checks are usually performed before the heating or cooling season begins. Insist on a written contract. When you have worked out the details, ask your contractor to put it in writing. This will help you avoid any misunderstanding of what equipment and services were included or hidden costs.