Any reason to get a table/countertop thicker than 1.5"?
Appearance and longevity. You can plane down the 3" one quite a few times, but it seems unlikely you'd be using cleavers on your desk. You might want to weigh the price difference between 6/4 and 7/4 against the ability to sand stains and dings out of the thicker one though. That extra quarter inch may add decades of life to the desk in the long run.