9mm carbines -- lots of choices and price points. So, lots for you to think about. I recommend handling them if at all possible.
I own the Beretta CX4 in 9 and 40. They are great ergonomically, relatively compact, accurate and lots of fun. Easily to switch for left or right hand controls and ejection. A plus is the easy and relatively cheap availability of Beretta mags. Some of the aftermarket mags are hit or miss, but MecGar work flawlessly for me.
AR in 9, I have the dedicated Bushmaster Carbon 15 (polymer). AR ergonomics, controls, and uses the Colt style mags which are (were) readily available and not too pricey. Disadvantage is the lower molded for these stick mags and can't use for anything else. Unless the mag profile fits.
I picked up one of the Just Right carbines in 40, yeah, not 9, but same principle. Uses Glock mags. AR controls for the most part (bolt handle is side mount). In theory you can swap barrels and switch calibers. One of these days I'll pick up a set for 9mm and maybe 45. (45 requires different mag well for 1911 mags). Again a dedicated AR.
There are a couple of outfits selling 9mm uppers to convert existing ARs. Most will require a different hammer or buffer. The lone wolf using Glock mags seems interesting but I already have the Just Right and Bushmaster.
Other options to consider
Ruger had a PC9 carbine some years ago. It was synthetic stock, light, handy and uses mags from the P8x series guns. I have one and it's a great shooter. Mags are easy to find and priced right. Ruger doesn't make them anymore but they are still out there and can be had for around 3-400. I picked up a second from a LGS for <200 (police dept trade-in).
The KelTec series -- they have those with either Glock or other mags. As mentioned above they fold into a compact package. I've shot several but don't own one, so I'll leave the feedback to those that do.
Similarly I don't own the HiPoint so can't comment on those either.
So, lots of choices, but a 9 carbine is a lot of fun and most of them quite accurate. And as Walkalong said --
Brass is free, bullets are cheaper than .40 or .45, it's easy to load, recoil is mild, shoots great, what's not to like.
Oh, to throw another wrench in the works -- I also have an MP5 and Uzi clone. But either of the ARs or CX4 goes to the range more than those 2.