This is the 64 million dollar question.
My belief is that it is time in the market that is the most important, then match this with the timing.
Ten years really is a minimum timeframe when looking at investing.
Most times it is very difficult to pick when a market bottoms out, so most people jump on board when the market is rising.
However, most people sell when they see the market going down.
The key is to buy for the long term, so you ride out the ups and downs, and have a successful, positive outcome.