They both mean something might happen or a certain possibility exists, and they are both widely used. One small difference is that -(으)ㄹ 수도 있다 seems to be a better choice when you speak with authority or when you''re taking responsibility for something. -지도 모르다 sounds more casual, like coming from a bystander. For example, if a doctor is telling the patient about the risks of surgery, they are more likely to use -수도 있다, as in 수술은 싶패할 수도 있습니다, rather than 실패할 지도 모릅니다.

fellow learner here. I would guess: 비가 올지도 몰라요 I don't know if it will rain 비가 올 수도 있어요 It may/might rain. Will look forward to a native response though.