A few years ago my daughter, her college room mate, and I did this hike. Because the room mate's feet were killing her she and I went down the shorter Mist trail. My daughter took the JMT. My daughter got down 30 minutes before us. Walking at 3mph is fairly easy along the JMT. The JMT is approx. .7 miles longer, but you can go way faster:-)
Yes, the Mist trail is far steeper and harder on the knees (it's a lot of steps).
I usually go up the Mist and down the JMT.
