If you are looking to invest money for the short term, then a more conservative mutual fund would probably be best for you. However, if you plan investing for retirement (hence retirement planning), a more aggressive mutual fund will be right for you. No matter which you invest in, absolutely make sure that retirement fund has exhibited a long and profitable history before getting involved with it. Past history is a good indication of future performance.
The bottom line is this: if you really want to reach your retirement goals, and reach your full potential with investing, then you should learn how to do it yourself. Nothing substitutes taking control of your finances and spotting investment opportunities on your own. However, if you don't have the time or the desire to do this, the Fidelity 401k might be a good option for you. Of course, there are many other companies that offer great retirement planning options as well; you simply need to do your research and find the best one for your needs.