Do you like plasticy guns or guns that look like guns? How much tinfoil do you wear and what percentage of your wardrobe is black? Are you an operator? How big are you? For reference, I am 6'2" and my shirt sleeve is 36" so I am on the large size of "normal." I have owned a few of these.
My recommendation is a Beretta CX4 storm if you don't mind the fact that it looks like a toy. There are also serviceable pistols that use the same magazine. My ADD doesn't let me load more than 10 rounds in a magazine anyway, so I don't care about preban magazines. The gun has little recoil, a modern design that has all the controls is the right place, and is more than accurate enough. It is easy to mount optics.
If you have a little more time to search, the Ruger PC9 is just as good and looks like a normal rifle. Same accuracy, easy to mount optics. Plenty of preban magazines available. More expensive than the Beretta if you can find one. Downfall is that the Ruger P series of pistols suck if you plan to operate from what I have heard, I have not owned a Ruger P series pistol.
The Keltec Sub 2000 is cheaply made and really is a 3/4 size rifle. I hated shooting mine since I was just too big for it. If you are 5' 10" or shorter, the gun will probably fit you well. The quality is poor which surprises me since the Keltec KSG is well made in my opinion. I guess it was designed and built to a low price point. Red Lion makes a lot of cool improvements for it, but some accessories impede the ability to fold the rifle. Mounting an optic is more or less impossible without killing the compact nature of the folded rifle. That being said, in the effective range of 9mm an optic really isn't terribly required anyway. I certainly felt like an operator pulling my Glock 17, 32 round magazine and sub 2000 all out of a laptop bag. Plenty of prebans are available and the glock pistols are some of the finest pistols you can buy.
Never bought a Highpoint, too ugly.
Never bought a Marlin Camp 9 since I haven't seen one for sale.
9mm in an ar-15 platform seems interesting, but the uppers cost as much as the complete rifles listed above. Never was interested in buying one of these.
I am going to buy a Mechtech upper for my Glock, they look pretty interesting and I hope it functions as a full size sub 2000 without folding.
Finally, you can get a 9mm conversion for a Tavor which I am considering, but it looks like a long process to swap over which negates the ability to swap calibers IMO.
Good luck!
Chris
![[rofl]](/xen/styles/default/xenforo/smilies.vb/013.gif "ROFL [rofl]")
is the same between the two. There is a thin plastic sleeve and a different magazine catch to swap between the two magazine types. 
