Deciding what mix of investments to use is what is known in the trade as asset allocation. There's been a lot of research done on this (nor surprisingly), and it repeatedly says that asset allocation is the most important factor ensuring suitable risk/reward balance in generating returns from a portfolio.
Which investments are best for you depends on the objectives of your investments. All investments behave in different ways, but they can be grouped into categories - also known as 'asset classes'.
Make sure your investment portfolio has a mix of these asset classes. This is a good way to ride out the ups and downs of the market. Which in turn will give you greater peace of mind.