If you are really a beginner with Arduino (and microcontrollers, embedded systems etc.) then buying the genuine Arduino is ideal. It works out-of-the-box with Arduino IDE (and rest of the software).
Once you gain some experience and confidence with Arduino, get to know all the basics, learn some more about the guts of Arduino board, you can definitely switch to boards that cost much less. Heck, you might even switch to an ATtiny85 microcontroller with few discrete components thrown together on a breadboard, that can take you far, for a tiny fraction of a full board cost. Or you could look at alternative boards -- Teensy, Jeenode etc.
Have no experience with sain, but have used a lot of east-asian goods (shipped from China, HK), and not once have I encountered problems. I buy off ebay, dx.com, seeedstudio and some not so famous places ! Of all of those, I prefer ebay for convenience, buyer-protection and low price. Seeedstudio stuff is actually reasonably good quality (better than a lot of what you can buy on ebay). However, as others (the gurus here) say, buying from someone who claims a board to be genuine Arduino, but actually isn't (i.e. are counterfeits) can leave you pretty frustrated.
Having said that, I am not sure if this discussion belongs in "Installation & Troubleshooting" sub-forum !!