My quick answer (and question) is whether you mean the larger, four separate burner Genesis II or the 3-burner with the extra 4th "sear" burner?
I had an earlier 300 series with the 4th sear burner. I agree with Mark that the regular three can do perfectly fine, but I did enjoy using the sear burner on steaks where I found reversing the order to using that side second rather than first worked best for me.
The "regular" 4-burner I am sure is a fine grill with more space for bigger cooks. I have heard ion a review somewhere, though, that it didn't fare as well for evenness as the 3-burner (or presumably the 3-burner with the extra sear burner).