The big difference between these two peppers is simply age. They are the same pepper, just a green jalapeño is picked early in the ripening process, while a red jalapeño is left on the vine to mature. During the ripening, the jalapeño, like other chilies, changes color from green to red.
Is a red jalapeño spicier than a green jalapeño?
It is. The additional ripening on the vine means more capsaicin in the pepper itself. Capsaicin is what gives hot peppers their spiciness.