[jfirecops]

For personal defense there is very little difference from the 9mm to the 40, shot placement is key. A conversions barrel will run around $100, that is around 500 rounds of 9mm. If I was making the choice, I would get the 17.

 

[jfirecops]

If you want to read up on the subject of caliber selection, I'd recommend the following:

http://www.firearmstactical.com/pdf/fbi-hwfe.pdf
The “Center Mass” Myth and Ending a Gunfight -Triggernometry
Thoughts on Service Pistols, along with Duty and Self-Defense Ammo Recommendations - M4Carbine.net Forums

A lot of good reading there, thanks.