both of these do have a big risk, but there is a difference between the two. I suggest before you choose one of the mining and trading you should have understood some advantages and advantages of doing so and we know that doing mining is in need of a large initial capital.
Here are advantages of bitcoin mining over buying bitcoins,
* Bitcoin mining allows their user's keep a track of their transactions, The bitcoins are untainted.
* There is no extra fee that is charged for acquiring the coin apart from the investment in hardware (this itself may be substantial though.)