I have not shot the 40 yet as my range only had the 9mm and I felt very comfortable with my placement with the 9mm.

I have read a ton on the debate between the two, but I was interested to see what ya'll think. The 40 might have an edge on stopping power, but I am a firm believer that placement is more important and that it will mostly come down to personal preference.

This is going to be my first pistol and I would also like my gf to be able to shoot it comfortably (this will be her go to gun in a defense situation).

Ammo cost is also a slight factor as I will probably be putting a ton of rounds through the gun. Is there any issue with 40 caliber being harder to get than 9mm ammo? Wasn't sure...

you can run +p or +p+ if you are concerned about stopping power, but the .40 is a very snappy round especially in a polymer gun. Additionally the cost of practice ammo is lower for 9 which allows you to get a better feel for the gun. In the end that will be more important than what some claim to be a "better" round.

Im also a shot placement guy.

And with rising costs of ammo, having the most universal caliber available is very important.

I dont think 40 caliber is going anywhere, but 9mm is cheaper and more available.

40 as S4ucy says snaps.

mr. fusilier has good points about ammo.

Main advantage of a 9mm is lower cost of ammo and higher magazine capacity.

trying to keep main ammo types....9mm, .45, .357, .38, and 12ga.

9mm or .45 ACP. I want to reduce the number of ammunition calibers I need.

I shot 9mm for several years because I also wanted control, shot placement and high capacity.

After a while I went to .40 and eventually to .45. I like high cap .40 for my carry weapon.

Add a Saigatech trigger guard, US made pistol grip, US made trigger components, a few AGP 10rd mags, and a couple MD-Arms 20rd drums.<ul><li><a href="http://www.centerfiresystems.com/sai-12.aspx">http://www.centerfiresystems.com/sai-12.aspx</a</li></ul>

had same questions - go with 9 or .40.
I went with 9, to learn control of the gun, as a learning curve weapon. My next handgun purchase will be a .45 or some other caliber as a small (pocket sized) carry weapon.
All the reasons everyone posted to go with a 9 are excellent reasons.

9mm for "comfortable shooting."

The likelihood that you will 1) shoot someone and 2) the 9mm will fail where the .40 would succeed is approximately zero.

The MD-Arms 20rd drum defines this weapon. What other 20rd shotgun is there that is not (yet) classified as a Destructive Device?

It does have one weakness though--lack of an automatic last round bolt hold open.