I've recorded with the XR18 and the QSC Touchmix16.
I currently own the Touchmix and I recorded last night's show. It worked great.
I bought the Touchmix because I can plug in a small USB SSD drive and multitrack record directly to it. I don't have to bring a laptop and manage a DAW to record... I just plug in the SSD, arm the right tracks, and press record. I throw the tracks in Logic after the show.
The XR18 will function as a computer audio interface, so it's better if you'd like to use it in that way.
I have a decent interface in my studio, so I didn't need that functionality in my live mixer.
I also like that the Touchmix has a screen interface if something goes wrong with the iPad... or if I don't wish to bring the iPad.