How many other handguns do you own? How many of them are in .40? How many in 9mm?
My advice is if you like the gun buy it. But buy it in 9mm, unless you've got some experience with both 9 and 40 and have an informed opinion. If this is a first gun, I'd rent the 9mm version and the .40 version. Then make your decision.
Got my ccw a few weeks ago and my g19 is just not comfortable to carry.