I think the Lower Cathedral lake is a nicer place to visit. It is definitely a nice day hike. It has a nice granite hump along the eastern side,and you can walk all the way around the lake, for some excellent picture-taking opportunities of Cathedral Peak. (IMO, one of the finest places in Yosemite.)
The JMT passes right beside the upper lake, but due to topography, hiking around isn't an easy task. It is a pretty place, and takes a bit more energy to climb up to it. Though for brisk hikers is also not a bad day hike.
