If you want the ultimate in low-light with high ISO & AF, 1DX, no question. This from a 5d3 shooter. The 5d3 is great, love it and wouldn't trade it, but every photo I've seen from the 1DX and what I know of it's AF/Metering system that's different from the 5d3 says it's even better. Especially with metering based on the AF selected, rather than just the standard modes that operate based off of the center of the VF. I'm so jealous of that.
If you just want to get shooting, and shoot a wide variety of subjects, the 5d3 is a great camera for that. No, not as good DR as a Sony sensor at low ISOs, but once you get up to the mid and high ISOs, it's as good or better. The AF system is outstanding as well, and the body handling & ergonomics are great. That said, if you think you can wait, the 1DX will be that much better for low light/concert photography.