Outside of trying the free A-Train: All Aboard! Tourism demo on the Switch, which won’t fully showcase everything the games have to offer due to it being limited to the first tutorial scenario. In terms of price, I would say A-Train 3D for the 3DS, since it regularly goes for 20 USD on the Nintendo eShop, followed by A-Train PC Classic, when it’s on sale at Steam or Humble Bundle.
Though, content wise A-Train PC Classic on sale is the winner if you get hooked like I did. There’s 23 official campaign scenarios that will each take a few to several hours to complete. So, you can easily spend over 100 hours just completing everything on the Easy difficulty. Boy, did I sink a lot of hours into that game on Steam.