Buy Directly from bitmain IMO, warranty can become an issue if you get it from someone else.
I have bought 4 S1's and 2 S3's from them , all worked great, no issues. S1 is still hashing (more so to prove out how much of a beast it is ) about 2 years it's been going.
It depends if you want faster shipping or not. Sometimes I will buy from websites like eBay because they ship from the US, not China, and I'm willing to take the extra cost for saving time. Warranty has never been an issue for me even buying from eBay or Amazon. The only bad thing is, people resell for a higher cost there, and if you're willing to buy it for more, but have a smaller ship time, this is for you.
Bitmain's trustworthy enough that you can safely buy from them, unlike BFL which ended up not sending it's miners at all for many people. For now, it is safe, but watch out , because as Bitmain becomes more of a monopoly this could change.