The SYN-1 would sit on a cab / power amp for live use and there is a footswitch to bypass or change channels. The IR-X would most likely be on the floor with no extra cable and footswitch. If you have other pedals that you are going to use, the IR-X could live on the pedalboard altogether and just be one or two less things to set up and tear down.
I am on the side of the SYN-1 though because I am not moving my rig around anymore, and I like being able to swap out the preamps for different needs. If I was gigging regularly, I can appreciate the more simple set ups and tear downs of today and if the IR-X had the tone I needed, would seriously consider gigging using that route. The SYN-1 would still be one of my main rigs though.