A flat white is a coffee with more of an equal balance of espresso and milk. The word "Latte" originates from the Italian word latte - referring to the milk itself.
A latte is made up of espresso, steamed milk, and some amount of foam comingled together. What you are asking about depends on three things - the size of the cup, how strong it is, and what you are looking for in your coffee experience. If sized comparably, a Flat White actually has over 20% more caffeine than a Latte. It also has less sugar while retaining more flavor.
Bottom line: the flat white is more caffeinated than the latte, but this fact does not make it stronger. The higher levels of protein in milk compared to espresso makes milk coffee inherently less strong.