Both amps put out a similar 800 watts @ 4ohms per side. The Crown is cheaper and can be bridged. QSC has a winner with their PLX line. The QSC has more of a fanbase here on TB but I don't hear many negatives regarding the xti's from those who have one. Will probably be running an Ashdown preamp with a pair of 4ohm cabs.
I have owned both and I still have the QSC PLX 1804. I had some major issues with my Crown amp and even had Crown send me a brand new one that still had the same problem - the digital front face kept changing my setting no matter what I did and the second channel was unable to accept a signal. The QSC works great for me - very clean sounding. Not one problem with it yet. Very much a plug and play design.