They are the same chip but a different stepping. When yields get better they also tend to be able to get a better stepping in with bug fixes and better thermals as well.
When Intel released the Q6600 it was a 125w TDP CPU with the B3 stepping. They worked out some bugs and some thermal spots and the G0 stepping was a 95w TDP CPU. And you can use the same mobo for both CPUs.
I would go with the 95w TDP. It will run cooler and also be able to OC better while also giving about 1-2% better performance due to the later stepping and all at the same price.