I agree with semi, with the exception on which one fits your hand. The .40S&W and 9mm in many makes have the same grip and frame. Glock 19 vs Glock 23 is one example.

Some people do not like the snappy recoil of the .40S&W. I would suggest going to a range and renting a couple of 9mm and .40S&W pistols and see which ones you enjoy shooting.