Same panel different implementation. Differences will be very small if not negligible and they would be in the realm of out-of-the-box color/gamma, overdrive modes and input lag. Check reviews if you're that worried but it will probably just come down to which monitor has your preferred aesthetic.
As others have posted, they are very similar. I went with the Alienware and have been happy with it for almost 2 years. The nice thing with Dell is that they will send you an advanced replacement and you can ship your monitor after receiving the new one. My first AW had really bad IPS glow (about 1/4 of the screen would turn orange) but replacement screen was almost perfect. Paid $1000 for mine and have no regrets to this day. I only wish it was freesync so that I'm not tied to Nvidia GPUs