Go with EWR. Rental cars are right at AirTrain station. Easier to get to.........however stay away from rush hours.....either airport during those hours 6-9AM, 4-7PM will take you a lot longer than a NY minute to get there. If you are going to SLC, you shouldn't have a problem as you will get to EWR b4 the rush.
Lincoln/Holland Tunnel to NJ Turnpike right to EWR. No traffic, about 30 minutes.