I expect that travel time by car from either LGA or EWR to the Conrad would be about the same -- but always subject to the vagaries of traffic. You can check Google Maps to confirm. Taxi/Uber fares from EWR will be somewhat higher.

By public transportation, the cheapest way from EWR would be the #62 bus to Newark Penn Station, followed by the PATH train to World Trade Center station. The hotel is 4/10 mile from there.

From LGA, you would take the free Q70-SBS bus to Roosevelt Avenue/Jackson Heights subway station, and then a Manhattan-bound E train to the E train's World Trade Center station, followed by another short walk to the hotel.

From either airport, it would be a single transfer -- from bus to rail.

On Saturdays, the E train runs every 12 minutes, while the PATH train runs every 20 minutes. But the ride on the PATH train would be ten minutes shorter.

Maybe fly into LGA, and out of EWR if you want to try both?