In this limited scenario, flex will probably always be watering, since even if it doesn't have to (let's say it rained the day before), it will look forward to when the next time it can water, and will determine that next watering is 7 days out, and will more than likely water.
Flex might be safer than fixed, especially if you have weather intelligence (virtual rain sensor) enabled. Fixed doesn't look ahead, and might skip your schedule based on rain received today/yesterday but not taking into account next watering.
The watering times are difficult, since a zone can only absorb certain amount of moisture before saturation.
Maybe start with flex, see how it works, and if not desired switch to fixed?