Singing is talent based, you either can sing or you cannot so it's definitely harder.
Acting is more about losing yourself in the scene and grasping the mood and atmosphere to deliver a good performance. It can be easy but it all depends on how well you can adapt. I personally like it a lot.
Acting is harder for me. It needs much experience , and with more experience you get better since I can't say the same thing for singing. I find it innate.
Singing due to the fact that you have to promote each song you release, then the album, film music videos.. then you have to tour.... whereas with a film you just have to do a bit of acting and then do a couple weeks promo for it
Being a good singer is something you're born with or not. You either are or not, lessons can make you better, but it's inherent. Acting, can definately be improved through lessons and acting. Just singing isn't going to make you a better singer.