You want to be a more active investor (IRAs generally have the full universe of stocks, mutual funds, ETFs, etc. available vs. a more limited 401(k) investment lineup) and/or you're with a smaller employer that may not have very competitive investment/recordkeeping fees with the 401(k) vendor. If you're with a big employer with low fees and you're content to choose from a smaller number of funds and let the market do the work for you, 401(k) may be better.