Having owned eight 9mm carbines over the past 15 years including most that have been mentioned here I have some experience in these things, but I am NOT an expert. I currently am down to only five 9mm carbines having sold off the ones I cared less for
Which did I keep and why?
Beretta Storm CX-4 (Ten years) Stone reliable. Very accurate. Small, compact, well made other than the trigger...but that I fixed in 15 minutes. Never misses a beat, chrome lined barrel and loads of fun.
Marlin 9 Camp Carbine Also stone reliable. Old school carbine roughly the size of an M-1 carbine. Nothing breakes, old school steel receiver and more.
Hi-Pioint 995 TS Reliable. Accurate. Life time warranty and cheap. I gave $238 for this one. I actually got my first one back in the 1990's when they first came out. I lived in Ohio close to the distributor. Shot well and never gave me a problem. It is heavy and crude with a zinc bolt and very basic engineering. Small 10 round magazines. But it shot and shot. Ugly is only skin deep. Sold it years later but missed it so bought a new and improved 995 TS just to have one.
Just Right Carbine Looks like an AR, uses AR trigger and furniture but takes Glock pistol magazines. Very accurate and reliable
Oh, and I can't forget my Kel-Tec Sub9 (not the new cheaper S2000k) Folds up, very light, and shoots petty straight.
These are all good choices in my eyes, but, YMMV
TNW ASR (Aero Survival Rifle). 5 pounds of fun. Smallish carbine with a barrel that screws off my hand without tools and packs away. Uses Glock magazines. Price is not as cheap as a Hi Point but its a real gun with a steel bolt. Mine is a smooth shooter and hits where you aim it shot oafer sop.