Police departments are going back to the 9 so they can get higher scores on range qualification, that's what I'am reading.
I second this...
Trainer. Gun(s) / Caliber
Travis Haley Glock 17 9mm (also 9mm M&P)
Massad Ayoob Glock 17 9mm
Chris Costa S&W M&P 9mm
Larry Vickers Glock 17/19 9mm
Gabe Suarez Glock 17 9mm
Rob Pincus 9mm (recommends a variety of handguns in this caliber)
Paul Gomez (RIP) Glock 17/19 9mm
James Yeager Glock 19 9mm
Andy Stanford Glock 19 9mm
Kelly McCann Glock 19 9mm
Jason Falla Glock 17 9mm
Michael Janich Glock 17 9mm
Dave Spaulding Glock 19 9mm (sometimes Ruger SR9c)
I own all 3, that being said there are many reasons for going with each different caliber.
Cost
Person size
Training issues
On that list I see guys running there classes with 9mm because it is more affordable to shoot 9mm than 40 S&W or 45ACP.
I prefer shooting 45ACP because it is more fun to shoot, but for training purposes or say actually real use it depends on the circumstances.
All guns are made for many different reasons and they are specific in duties and mission, the criteria may vary so there is no simple answer or one answer..