ETSs are all the rage – and they more closely represent the live market because they trade – so for better liquidity, buy ETSs. Assuming you have a 401K, you probably have the choices of traditional mutual funds – like others have said, go global. If you are years from retirement, don’t worry too much about it – pick a large fund manager – you don’t want the fund shutting down on you – and hope it goes lower or stays relatively cheap for the next 10 or 20 years – and then hope it appreciates for the 10 years leading up to retirement. Lower prices means more buying opps!
The market always beats the managed funds, so if you want to be able to buy and not have to watch it or worry, but an index funds. SPDRs for instance.