For Sure the Bus is Best
I think it's best to get your bus from Koa San road in Bangkok (Backpacker Area) into Cambodia.
It was easy for me and they picked me up at my Guesthouse so I didn't need to drag my bags too far.
You are from Thailand so crossing the border into Cambodia is probably a common thing for you to do.
I also prefer the bus because it is easy, cheap and efficient.
But something has to be said for taking the train from Bangkok to Aranyapratet. It is a very special way to see rural Thai lifestyle that tourist may not otherwise be able to see and experience.
No. For tourists my vote goes to the train from Bangkok to Aranyapratet.
Stick your head out of the moving train car and you're in another world, literally.
That's what most people want to see.
Regards for this.