service or sales scenario to be used for the mentioned case

hi all, i had a query. I have a scenario of transportation services with some key points in mind.

1. A contract exists between customers and service providers which lasts for many years.

2. Orders are created w.r.t. the contract over a period of time.

3. Billing is done in CRM and the order is transferred to ECC for delivery.

Now for the CRM part, is it a good idea to have a sales order and contracts or service order & contracts?? thanks in advance