If you want something free then Lite-C, Irrlicht or Ogre will be good choices. In any other case there is not much difference, coding is coding, you need to know the capabilities, the commands, the behavior. You have to do it while working on a project and you will learn it, like Samb or Kiyaku are doing. There is probably no other way to do it fast and efficiently.

At Dexsoft we will buy a license of Unity as well, since many of our latest customers are using Unity. So we can check our models in Unity ourselves.
And the good deal is: 2 people are allowed to work with one license. So we can share the expenses.