Well depends on what you do, if you are playing a lot of games, videos and something like high capacity programs then you should get a hard drive ofcourse but if you are not into games and using a computer for web and work then a single 120GB SSD is enough, I have a 120GB SSD since im not into games it is enough for me.
But if money is not a problem consider to get both. hard drive for data and ssd for boot.
For SSD's I recommend Samsung 840 Series or Kingston HyperX 3K SSD.