rowing machine if done properly (level 10, back kept straight, bar pulled to your solar plexus with each stroke, 30-35 strokes/minute) provides a far more intense workout. If you need any more proof, go into your local gym and see how many girls use the cross trainers, and then compare it with the number of girls you see using the rowing machine
I generally find I burn about 200 calories in 10 mins on the rowing machine, but it takes me almost twice that much time to burn the same number of calories on the cross trainer, with treadmill (running at 8mph) somewhere in-between.