I don't think you should just learn one; they are all used for different niches. It depends how your app trades off different things (consistency, reliability o

I don't think you should just learn one; they are all used for different niches. It depends how your app trades off different things (consistency, reliability of reads, reliability of writes, speed of reads, speed of writes, efficiency of hardware use, and so on). See http://news.ycombinator.com/item?id=2052852 http://news.ycombinator.com/item?id=2052852 for a comparison.