I love my Nexus, it's running Jelly Bean and running it very well. I did fall in love with it after rooting and flashing a rom. Today I get my GS3. So my opinion.....get both of them.
All kidding aside, this GS3 better be pretty good to beat out my Nexus. I'll come back after I play with the GS3 for a bit. I can say if you will be running stock, get the GS3