How Google picks which URL to display
Matt Cutts, the Google webmaster, has some advice about URLs:
"...most people would consider these the same urls:
But technically all of these urls are different. A web server could return completely different content for all the urls above."
And the best way to ensure that Google picks the URL of your choice is by using that URL consistently on all of your web pages. If you provide Google with only one URL then Google will likely provide the same URL in its search results.