PeerSync does not provide for a priority-based system, however you can achieve a similar result by restricting the resource usage of the jobs you want to be considered as low-priority.
Under the 'Performance' options for each job, you can limit the amount of bandwidth used for that job. If you are currently using the multi-threading feature, you can limit the amount of copy threads that each job will use on a job by job basis by limiting the 'Set maximum number of Copy threads to' number for each job.
By limiting these performance options for the lower priority jobs, you can ensure that the high priority job will not have to wait for resources to become available.
Also, as an alternative to limiting the performance on the lower priority jobs, you can configure the high priority job to perform its replication in real-time, and have the lower priority jobs configured to perform scans on a scheduled basis.