Index funds usually have lower management fees since they're not actively managed. Also, they trade much less, so turnover ratio is lower (making capital gains taxes lower).
Of course there are mutual fund managers who may outperform or meet the market indexes (DODFX got amazing result during the last five years). As there are other managers who can barely keep up with market indexes or not even that. However, I don't trust people and you actually have to do your homework before choosing a manager.
Index funds tend to outperform mutual funds in the long run. I feel it's a safer option.