This is a wrong comparison but this question is asked often – OmniPUSH is a automation tool which works on top of Linkedin Search.
Sales Navigator provides more granular search fields like – Position(CEO, VP, Manager, etc.), Employees (5-10, 10-100, 100-1000, etc.). If you are targeting Professionals like Dentists, Attorneys, etc., You may not be benefited from such granular search. However if you target companies, you may find Sales Navigator useful. It does not provide automation capabilities though which are exclusively available in OmniPUSH.