Others have talked about the differences in the panel (WLED versus GB-r backlight). But there is also a difference in the electronics - where the P2415Q is superior. The older monitor must be driven as two logical displays over MST to run at full refresh. It appears to the video card as two 1920x2160 screens and it is up to the video driver to tile them back again. Not all video drivers can do this correctly all of the time, or they may do it but not support rotation. That said, the driver support has improved since the monitor was first released.
Additionally, with Nvidia cards, two logical outputs are 'used up' by the MST arrangement, so a card which supports two screens will only be able to drive a single UP2414Q over MST, even if there is a second DisplayPort physically present on the card. (This is the case for the NVS 310 for example.)
You can run the UP2414Q without MST, so it more straightforwardly appears as a single screen, but then refresh rate is limited to 30Hz. If you want to switch to lower resolutions (for gaming perhaps) then you need to switch off MST in the monitor's on-screen menu.
By contrast, the P2415Q can be driven as a single screen at 60Hz.
There are also some firmware bugs with the UP2414Q, particularly related to waking from sleep mode. I had to install the Windows utility Don't Sleep to work around it. I think the newer firmware releases (A01 and A02) may fix this, but the firmware isn't upgradable and you don't know when buying a UP2414Q from Dell which firmware revision you will get.
So it's annoying that there is no single best choice. You may consider the NEC EA244UHD-BK, which I think combines the UP2414Q's panel with superior electronics, but it costs more than either of the Dells.