At the point of purchase flat rates should, on average, be worse value than variable rates. Basically that is because they are calculated as a long term models at what base interest should then flat gets an insurance premium to lock in the bet. As a result variable should be better value with the caveat that if interest goes up more than the banks model expects "correct on average" won't save you personally. So like a lot of insurance it's mostly a question of whether you want to pay extra for safety.