First choice is better, because google will treat the "-" as space, and see word1 and word2 as separated keyword. But it doesn't help too much, google can also read the "_".
ain, how do you measure that '-' and '_' is equal ? is it a rules or based on your experience ? thanks
Based on my personal experience, '-' is much better than '_' for the URL. I have also found that the character used on subsequent pages (X_X.php or X-X.php) does not seem to make a difference.