Options for JFK to EWR transfer

Google is giving me public transit times ranging from 1h 30min to 1h 55min for a trip from JFK to Newark, arriving at 5:30 pm this Sunday.

If I were you I would look very seriously at the latest allowable arrival time in Newark. If you have checked bags, you will have to be there earlier.

Checked bags will also delay you at JFK. In my experience, no matter how long it takes to get through passport control, I have to wait to get my bags. Baggage handling is therefore the limiting factor for those with checked bags (this may not be true for those in the non-US immigration line.)

Finally, your first flight might be delayed, so you should assume that you may miss the plane no matter how you make your transfer. You should only fly this itinerary If the airline will rebook you for free in the event of a missed connection.

The train route is longer and requires multiple transitions but is much more predictable for time.

ETS could be faster and you have no connections to worry about getting to EWR. If you go with ETS, I would recommend an earlier shuttle, there are way too many pinch points for traffic on the route from JFK to EWR. Backups at the bridges and traffic through Brooklyn and Staten Island could easily double the trip time. That could be a risky proposition when you have a specific time you need to get to EWR.

Predicting time in customs is hard. Too many variables, time of day, number of international flights arriving.

YMMV, this is my experience living in central NJ and having sat through way too many traffic jams heading to/returning from Long Island, a trip which covers most of your ETS route from JFK to EWR

The connection time is obviously very short for a busy day, no matter how you go.

If you want to keep a flight like this, like if it was a very good deal, and you're flexible time-wise, you might try and better understand what will happen if you misconnect. Investigate later connecting flights you could take if you miss the first one. Remember it's a busy travel day so consider how frequent they are. Contact the airline, preferably via email so you have a record of it, to find out their process for switching airports, will they blame you if you don't make it in time and cancel your connecting flight stranding you in Newark? Will they confirm you on the next flight out instead? Will they'll pay for your accommodation overnight if you can't get out later that day and have to wait for the next morning?