I personally find the cross trainer much easier then the rower (yes, it depends on your intensity) but my heart rate is much higher going easy on the rower compared to going easy on the crosstrainer. Then again, you can probably stay on the crosstrainer longer. I usually do about 2000metres on the rower (about 8 or 9 mins) and its quite hard. Then I will spend the remaining time of my workout jogging on the tready.
I usually tell my clients that the more weight bearing exercise you do the better. Eg. walking, jogging, skipping, even using dumbells instead of machines. The non-weight bearing exercises such as bike, crosstrainer, rower, using a leg press machine instead of squats with bar or dumbells etc, etc are good too, especially for beginners or people with back/knee injuries but its up to you and how much intensity you want to use in your workouts. My own opinion is that weight bearing exercises may burn more calories, help with bone strength, are better for working your core muscles and good for posture. Hope that helps.