The Insurance Company responsible for your claim must pay for the cost of an equal replacement vehicle or provide you with a replacement vehicle. It is their choice. (refer to Utah State Law)
The Insurance Company may offer you a dollar amount for your vehicle (less the pre-agreed upon amount of your deductible). If you do not feel this is an adequate amount, you will need to substantiate your claim for a higher dollar amount with independent written evidence showing the worth of a similar vehicle (year, model, mileage, condition and options) in the same market area. The market area may be local or regional depending upon the number of vehicles for sale in the area.
Remember, upon extension of their offer to you, the Insurance Company has fulfilled its obligation and can at that time stop payment for rental car expenses. If your vehicle is totaled, visit your local dealerships or go online and obtain information that substantiates your claim of what you believe your vehicle is worth.