Is it easier and cheaper to buy your one way ticket in advance or at the station on the day.

It's the same costs whichever option you choose.

if you are using Amtrak, buy in advance. other trains NJT same price.

Of course

Easier to buy in advance. I would use the kiosks preferably, or even the app, as the ticket agent lines can be fairly long and time consuming at times.

Yes.

Buying a ticket in advance is always cheaper.