RE: UP3214Q/UP2414Q - 1/2 screen issue and/or No picture after powering off, then on
Actually, the display is capable of running 1080p on the full screen just fine - intel's latest drivers for the Haswell graphics unit do it (I personally verified). So, that seems to be a limitation of the NVIDIA drivers. It is already under investigation - I opened a ticked on lower resolutions a while back and the 344.11 drivers were supposed to fix the issue - perhaps they did for the Asus screen, but not the Dell (I only tested on an A00 though).
As for 2560x1440, the display doesn't seem to report that as a supported resolution in MST mode. The Asus screen seems to be running an older scaler firmware that cannot handle this resolution - whether it is the same for Dell I've not been told.. they're now looking into it.
So, for 1080p, the ball's in NVIDIA's court, for 2560x1440 it's anybody's guess (my guess would be that they probably did the same as ASUS.. skimped on upgrading the scaler firmware - that is a fixable issue though given Dell's track record with these dual tile screens I wouldn't hold my breath).