I think it depends on your family circumstances. Having a pre-pay plan would mean you would have at least SOME say in what happened, as you would have chosen what to pay for.
My parents and my parents-in-law all have insurance policies which were intentionally set up to help with funeral costs (unfortunately, 2 have been used

), but they were (are) confident that we would see their wishes were followed. Similarly, DH and I have insurance, and DD knows what we would want by way of funeral arrangements, and we know she will follow those wishes.
However, if you are not confident that those making the arrangements would know, or respect, your wishes, maybe pre-pay is the way to go.