You will need to prove their income at time of application (depending on the amount of the monthly benefit), which has the advantage of showing what you will receive if a claim is submitted, regardless of any fluctuations. Unfortunately, an agreed value policy attracts a higher premium. This type of cover could be seen as more suitable for self-employed or occupations that see their income fluctuate.
Your income at the time of application will determine your monthly benefit and the premium you pay. However, when you submit a claim, you will need to provide your pre-disability income. Your payable benefit is dependent on your highest average monthly income over a defined period of time prior to your disability. This type of cover could be seen as more suitable for regular salary earners i.e. office workers.