Trade into momentum and strength. "Go with the flow", "The trend is your friend". I know these are pretty old sayings from the market, but when I follow them, I personally have more success then when I don't.
Also, using TA indicators, it's a LOT easier to pick a move when it's already happening and signalled it's intention than to guess when/where it might.
Trading weakness (which I used to do) smacks of trying to pick tops and bottoms, which, although profitable when you get it right, you don't get right that often (or at least, I didn't).
There's room for both I imagine as long as you work the probabilities and risk assessments correctly. Otherwise, it's just gambling.
Grey1 - for you to bring this question up under the "Pair Trading" heading leads me to think there is a purpose - other than the 'obvious' one I've just responded to - which is specific to this trading strategy - so I'm very keen to find out where it's going...