Hi Takehiko!
First of all, I think you should decide what kind of person you are: biased or unbiased
I mean: there's a lot of difference between the two kinds of approach, and you'd better choose which one you're more familiar with.
Biased rendering are less "physical correct" but faster during the calculations, but they usually take longer to prepare the setups and adjust things. This is the case of Vray, which is an EXCELLENT renderer engine. It's totally worth its cost, although you may need some time to understand it fully.
Unbiased renderers have less setup time, and longer rendering times, but the result is more accurate. But, based on the fact that they reproduce the real world, you can hardly "cheat": you must think as a photographer, more than an image creator.
Given those considerations, you will be able to choose the right renderer.
Still, you can download all of the demos you want, test the different workflows, and see what you like most.
Keep us up-to-date and good luck!