where you wait for other people to choose where they want then you select later. when you are the first one to book, you will choose the best place to sit.
if you will have such an assumption, you will definitely go for the cheaper option. the cheaper option may not be the best place for you to get the best seat. therefore you have to assume that and explore all sections of the plane and choose where you are comfortable. see here for tips for choosing cheaper flights. 