Ben is right, both are important.
Skills assessments determine if a person CAN do the job, behavioural assessment determines if a person WILL do the job.
I am a Behavioural Expert and train organisations on Behavioural Interviewing and Assessment. As each job requires different behaviours, just as it requires different skills, the assessment must be tailored to the requirements (behaviours) of the job. Managers need to have a clear understanding of the job requirements and key behaviours if they are to gain benefits from behavioural assessment.