That's a hard choice, but in a day you'll have to pick one. The Mist trail should be open all the way to Nevada Fall. The section of the John Muir Trail between Nevada Fall and Clark Point (where another trail cuts down to the Mist trail) will probably still be closed, but that lower part is an option if you didn't want to return on the Mist trail the whole distance.
The base of upper Yosemite Fall is pretty spectacular, and the view just below it, near Columbia Rock, is fantastic, which makes it a hard choice. Both trails have their difficulties; the Mist trail you'll want some rain gear, a cheap poncho or something, and/or quick dry clothing. An extra pair of socks is nice to have with you if your socks happen to get wet on that trail
http://www.pbase.com/roberthouse/image/97758180The steps are numerous and steep,
http://www.pbase.com/roberthouse/image/61297012and can be hard on the knees coming down, which is why sometimes the John Muir trail back is an option. Very scenic route though, all the way to Nevada Fall.
The Upper Yosemite Fall trail starts out in the shade (nice) but is an endless series of rocky switchbacks, ending at Columbia Rock about a mile up. That's where most people turn around, but if you trudge on a little way past some barren new trail, you come to the base of Upper Yosemite Fall, which is a great sight.
http://www.pbase.com/image/65194704Overall, with only one choice, I'd probably take the Vernal/Nevada Falls hike, as it's a steady supply of amazing sights the whole distance. If you start early it will be more likely to be a pleasant uncrowded hike.
For something lighter in a different part of the day, you might enjoy a walk through the Happy Isles area; there are trails and bridges over the amazing Merced river, which should be fairly raging this time of year, and it's an easy walk but very nice.
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo