The caffeine strength of both an Iced Americano and an Iced Latte will depend on how many shots of espresso (approx. 120 mg caffeine per shot) go in. Typically, an Iced Latte will be made with one shot of espresso, so it will be less strong, but there are plenty of people who ask for two shots.
Although the caffeine content will be dictated by the amount of espresso used in either drink, Iced Latte benefits from the addition of milk (dairy or alternatives), so it tends to taste sweeter and less bitter than Iced Americano, which is served black.