It should all depend on what the agreement is being made about. I hope we are not talking about an agreement that involves a medical device creation. If this has to do with a medical device I will strongly suggest the contract be written in that medical devices often involve the human. The format of other types of contract agreements can be dictated by the value of what is involved and the reliability and trustworthiness of the contractors. To be sure what is being contracted has no complications in the long run, I will just make life easy for me – I will demand a written contact. It’s easier to work with a written contract than with a verbal contract; unless, of-course all involved are family members who will be unlike to make trouble later.