ETF's 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. 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. But you cannot tell purchase price until after you buy. And shares may have to be held for a required period before sold to avoid a surrender charge, so may only be suitable as long term investments. Mutual funds with a high turnover rate may have more capital gains than holding onto an index ETF.