I am the odd one out. I enjoy the Marksburg, but like Burg Rhinefels a whole lot more. Have been to both of these castles at least 10 times. Marksburg is the luck of the draw as to whether or not you get a decent guide that you can also understand during your 40 min. tour.
To call Burg Rhinefels just a pile of ruins is rather incorrect. Climb the towers, enter the great hall (which takes my breath away every time I have been here) explore the tunnels, walk the walls and you will realize that it certainly is not just a pile of ruins. You can stay there as long as you like and explore on your own. Kids, especially teens would have a lot more fun here than they would at the Marksburg. Take a flashlight or make sure your phone has a flashlight ap on it. Make sure to use the toilets at the castle too. They make me laugh every time.
Is there a reason why you can't go to both of them? It is easy to see both in one day.