Pistol ARs are not to be dismissed. As for .300 vs 5.56 you must ask yourself. Can I afford to shoot it at $1 a round? I like the .300 for its niche of 100-150 yards suppressed with the ability to do 200 unsuppressed and supersonic. The issue I have is I can’t afford to train with it or get to know it the way I know my 5.56 guns. If this is your only carbine it’s an even more important question. I bought a 9” barreled pistol with Law Folder so that I could have a super small footprint for Shooting suppressed or traveling. It just takes up the least room.
If you can afford it then consider it within its parameters. If it works for you it works. If not and it’s 5.56 there are guns out there that run well at 10.5”. They are however a more self destructive system than even an 11.5” barrel that is just 1” longer. The longer barrel you can stand, the better off you are in the long run durability wise. Longer barrel life less cracked bolts etc. The short sweet spot for me is the 11.5 to 12.5 range. The only gun I have under 11.5” is a Mk18 but it was just a fun gun not a work gun.
With 10.5 and 11.5 guns you need to note that the loss in velocity can affect your ammo performance. So for self defense or hunting you will need to select an appropriate bullet. 55 grain ball isn’t gonna cut it especially out beyond about 75 yards. Consult the ballistics to see what your barrel does at you prescribed max range then find the bullet that performs at that velocity/range. 70 grain Barnes TSX, 5.56 pressure Trophy Bonded bear claw such as the Federal Tactical Bonded FBI T3 load, Etc.
As far as brands go BCM makes good ones in both calibers, Daniel Defense has a nice Pistol AR, SOLGW, FN, etc. There are lots of good ones to choose from. Do you research, if they make a great 16” AR odds are they make a good pistol too.
Pistols have the legal upside of being pistols which often allows for additional allowances in the CCW realm. You may be able to conceal one on your person or in your vehicle depending on your local laws where you might not be able to do so with a rifle. Give serious consideration to the Law Folder. It goes with AR pistols like PB&J. It’s available from Daniel Defense frombthe factory.
There are no free rides with either caliber or barrel lengths. Everything is a sacrifice for length. If you define your explicit needs or your bare minimums it will make the choice easier.