- ETFs, Vanguard is a good choice for most. If you're older and might need a large percentage of the money fairly soon, consider getting some bonds as well.
- Don't try to time the market
- Don't think you're smart
The only personal difference is I prefer FTSE All World as it is diversified into over 4000 global stocks, while the S&P 500 is (obviously) 500 American stocks. That being said the S&P 500 has been outperforming the FTSE All World for a long time, and I certainly don't want to give anyone specific investment advice.