Always use a letter of intent to present your preliminary offer to a seller. This will save you writing time, legal fees, and will be easily read and understood by the seller and any other interested parties who must be made aware of your intention to purchase the subject property.
A LOI is typically used as a "starting place" for negotiations on a property. Generally a LOI will be followed up by a formal Purchase Agreement after the terms are settled on. This may or may not be the case in your area, but is a good use of the two documents.