Actors I guess because they are more watched in TVs.. Singers are usually famous to music lovers only but actors are usually famous to almost all types of people.. But for both, only so talented actors and actresses get the attention of the audience..
Singers are more popular because they are always heard on the radio or when anyone plays their songs anywhere. You also see them on MTV regularly while actors are just seen when they have guestings on TV shows or when they have a movie shown.
According to me Actors are more popular. Singers are very rare which are popular but the actors are almost popular.