In most cases, an Americano will be stronger than a Cappuccino. A standard Americano, made with a single shot of espresso and hot water, will have 95-150mg of caffeine while a Cappuccino, made with espresso, steamed milk, and milk foam, has around 75-77mg of caffeine.
This value can vary depending on how many shots of espresso are added, but either way, the Americano is almost always stronger.
The strength of the coffee also depends on the origin of the beans, the way they are roasted, and their freshness.
A strong Americano can be quite bitter and difficult to drink so many prefer drinking multiple Cappuccinos to achieve the same level of caffeination, rather than drinking an extra-strong Americano. Both these drinks tend to be quite strong either way, so go with what you like the best!