Dell's specs show the old model has marginally higher peak brightness 350 vs 300 - which is irrelevant unless you're going to be using it in a location with extremely bright lighting. Along with the lower power consumption it probably indicates a new model panel is being used.
The newer model supports DP1.4 instead of 1.2, which I don't think matters; and can provide 90W of power over USB-C. If you're using a USB powered laptop this would let you have 1 less cable to plugin.
The new one also has built-in ethernet and faster USB ports. I don't think it's worth it, but some did want the ethernet for a true one cable solution. I'm currently looking to "downgrade" my Alienware 38" to one of these since I don't game anymore.