Hands down I would say Amazon, (the 'sponsored pruducts' ads, not so much the 'product display' ones) and that's just my personal limited experience. Goodreads is great for exposure, but there are fewer users clicking on ads, and it's impossible to know whether you're getting any return on that investment. The one good thing about Goodreads however, like other social media, is that you can set the parameters for who sees your ads; on Amazon, you don't get that control. Goodreads ads are great for when you're doing a Goodreads giveaway.
Goodreads users generally are looking for the word-of-mouth books, and money doesn't buy that well. It seems that Amazon is where people are looking to actually make a purchase. Interesting as well, you don't actually need a single review to sell books. I know this because I sold around 20 copies without one (not to say it wouldn't help a bunch), but what you do need is an absolutely killer blurb, one that just hits people in the face, and of course, a mind-blowing cover and sample chapters.
Keep in mind, I wouldn't expect any positive return on ads regardless. Success is basically if you make your money back. Good luck!