A study in excellence
This is another topic that is frequently discussed. A search will provide you with plenty of reading.
I have owned the High Point, the Ruger PC9, and the Colt AR15 in 9mm (factory 9mm gun, not an upper swap).
I sold the other two and kept the AR15. My second choice would be the High Point.
As was mentioned, the AR15 is one of the most popular guns out there, so there are lots of goodies to add on if that floats your boat.
I started out with a standard factory 16" barrel. I then bought a 11.5" upper. Finally I bought an LRM M169 suppressed upper.
The 11.5" upper is one of the funnest guns I own. I have a KAC RAS II rail on it, and Aimpoint ML2 optic and a short Sully stock. This is the perfect carbine for shooting jackrabbits out of the truck. When out shooting jackrabbits, I add a Surefire 900 series weaponslight on the rail. The LRM suppressed upper also has a rail so I can swap all the goodies from one upper to the other if I want:
http://www.lrmfirearms.com/suppressors/m169.html As a totally unrealated side note: I chronoed some ammo out of this suppressed upper and it was actually SLOWER than the same ammo out of a KelTec P11 handgun with a 3" barrel. :banghead:
I also recently bought a 100 round Beta mag for it (which is only made for the Colt).
http://www.impactguns.com/store/BETA-CCS900.html The Pro-Mags also run flawlessly in my gun and lock the bolt back. Of course the factory Colt mags are also trouble free. I also bought a dozen unknown SMG magazines that work but don't lock the bolt back. Here is a little video on the Beta-C mag FWIW:
http://www.impactguns.com/store/beta.html
And here is a good article about the Colt SMG which is the select-fire version of the Colt 9mm that us peons can't own (or at least I don't think so, I have never seen a transferable one, although you could make one from a registered receiver........)
http://www.chucktaylorasaa.com/coltm635.html Even though it is about the SMG, most of the information applies to the semi-auto version as well.
I don't know if this has been mentioned or not, but if you go for the gun that takes Glock magazines, there are tons of those 33 round Glock 18 mags at every gunshow I have been to lately. Perfect for the carbine. Natchez has them on sale right now real cheap.
FWIW, I own three 9mm Glocks: 26, 34, 17 and plan on getting a 17L. That isn't enough to make me get rid of the 9mm AR for one that takes Glock magazines.