I'm thinking on buying a USB bit coin miner. And searching in amazon I found these two products, that I think that are the same but I'm not sure because one it's 15$ more cheap than the other.
So, what's the difference?
According to the specs, there is no difference between the hardware -- they have the same hash rate (330 MHz) and use the same amount of power (2.5 W), so they should be interchangeable. It is possible that there are software compatibility issues, but this is unlikely. The machines are likely identical, so there may be other reasons for the difference in price.
The value of pretty much anything related to bitcoin is volatile right now, including mining equipment, so it's reasonable to see fluctuations in prices like this based on the whims of individual sellers that don't reflect a broad trend across the market. Consider this: the seller with the higher prices also advertises a very limited stock. It's possible that both sellers started out at with same price, and while one decided to start selling off a large quantity at a discounted rate, the other decided to stick to the original price and just sell off all they have left.
So, since there's probably no real difference -- just buy the cheaper one.