I haven't used either. I have used the FS model G22, and the G27 subcompact. Both are very easy to shoot, much easier than I had expected. Obviously the .40 has superior ballistics to the 9mm, but I'm inclined to purchase a G19.
Why? Capacity and affordability. The ammo is much cheaper, and 2 more rounds in the magazine could be a life-saver. Remember, shot placement is most important. A well-placed shot with a 9mm is no less effective than a well-placed shot with a .40.
I feel I'll be more likely to make a well-placed shot if I am able to practice more. 9mm is significantly cheaper than .40, so I think the 9mm takes it.
I'm in the market for a G19.