ASUS VG248QE, the performance is almost identical, it’s so close when toms hardware reviewed then both they said there was no way to tell a difference, its that close, it is also compatible with G-Sync, you can get a DIY kit and install it yourself, and is cheaper!
I was looking at the same monitors before I found the ASUS, and I was going to go with the XL2420Z, just make sure you get one with Firmware version 2. Or you are going to have a lot of issues, and from what I have read it’s a pain in the ass to update it yourself.