HP Omen X35:
1440p
100Hz
VA
630€ / $715
Alienware AW3418HW
1080p
160Hz
IPS
780€ / $885
Actually, I am using (1 day really) the HP Omen X35 and it is not convincing me a lot.
1440p, really I think I can't see difference respect my old 1080p monitor, I just got lower performance in games
100Hz, it is amazing after using 60Hz for a long time, even in the windows desktop, using 60 fps now ugly.
My old panel was a Samsung, I think it was TN, but the colors of that panel are better than the HP Omen. I tried to replicate them using NVIDIA panel but they are still different.