I was really disappointed with the Galaxy S3. I will most likely be upgrading to the HTC One X. In almost every situation the One X beats or is a tie with the S3.
Screen: Most reviews say that the One X has a better screen then the Galaxy Nexus, so I believe that with the S3 having essentially the same screen as the nexus the One X's will be better.
Processor/SOC: The S3 will likely have an S4 in Canada so in that case it is a tie. If the S3 does have the quad core in Canada it will probably not be that noticeable of a difference.
Software: Both will likely have great Rom development, but in a worst case scenario, I can live with Sense 4.0, I cannot live with that hideous touchwiz.
Hardware: (IMO) The One X is gorgeous. The S3 is meh. Also, the S3 has a menu button, what were you thinking Samsung! I don't care about a removable battery or sd card slot.
Camera: I'm guessing it will be about the same (Don't care much about camera).