ETF's usually follow an index and can be traded quickly, any time during the trading day. They usually have a lower expense ratio, but a stock trade fee for each buy/sell. Watching ETF's too closely may be a disadvantage if you panic sell on what turns out to be a temporary dip. On the other hand you may be able to pick up some instant bargains "if" they bounce back quickly.
Mutual funds with no trade fee or load may be better for dollar cost averaging (regular contributions). But you cannot tell purchase price until after you buy (priced after market close). And shares may have to be held for a required period (30-180 days) before sold to avoid a surrender charge, so may only be suitable as long term investments. Mutual funds with a high turnover rate (frequent stock trading) may have more capital gains (even if NAV drops) than holding onto an index ETF (not an issue in an IRA).
Just be aware that there are many indexes that have very different performances, and only a minority of mutual funds beats their closest related index. So you have to do your own research before selecting specific ETF's or mutual funds.
