Unfortunately the sunscreen in makeup is not as effective as regular sunscreen. For the sunscreen in makeup to be as effective as regular sunscreen you would need to apply A LOT of it. Usually makeup sunscreen is under 10 SPF and you really should have at least 15 SPF everyday and 30 SPF when you are going to be outside (rather its raining, sunny, cloudy, snowing etc) for a longer time.
I had this question in esthetics school, because I always thought makeup sunscreen was enough. But unfortunately you have to take that extra step to ensure the protection of your skin!