I’ve just finished an article about exchange traded funds. If you have enough money, get an index mutual fund instead. Transaction costs will sometimes be lower with an index fund versus one that requires a trading commission. BUT, if you are a micro-investor, say investing very small amounts as an alternative to putting a thousand bucks into a low interest rate saving account, you can sometimes buy ETF’s more cheaply. Then you can avoid the low balance fees you’d get hit with if you’d bought a index mutual fund. Also, one brokerage lets people buy ETFs efficiently. ETF are also cheaper if you are outside of the USA and don’t have access to US mutual funds.
In the end, you will still have to sit down with Excel to dicide how best to invest your $100 a week.