After reading all the recommendations, I'd have so say you need to get one of each!
The 19 or 23 seem to be the best "all-purpose" choices, however. If you're planning on shooting 40, the 23 will give you that as well as the option to shoot other calibers with different barrels.
This is another good point get whichever model you want in .40s&w and just get a 9mm conversion barrel. Now you have both..