I have ARs in 5.56, 300 BLK and 7.62 x 39 in carbines and a 300 BLK pistol. I bought one in 2007 and put the others together over time. I am a deer hunter and normally hunt open country with bolt guns in 257 Wby or 30-06.
Since I built the AR in 7.62 x 39 I did take it once but did not get a shot and later on the same hunt killed a deer at 282 yards with the 257, so it only goes when shots are 200 yards or so. I did kill one large deer with a mini 30 and got sold on that round.
The last I put together was the 7.62 x 39 carbine and it quickly became my favorite. If I could only have one it would definitely be the 7.62 x 39 for many reasons. With 150 grain bullets it is far superior to the 300 BLK. But that really only matters if you plan to hunt with it. You can still buy steel cased ammo which groups about 2 inches in my guns, at 40 cents per round and countries other than Russia are making it now. I buy it and shoot it just because it is that cheap and likely always will be in my lifetime.
If hunting is primary use, then I would suggest the 350 Legend as Wicked109 says. It is superior to the 7.62 x 39 and takes all the 357 pistol bullets plus rifle bullets up to 180 grain. That makes it a thumper for anything you might want to hunt. You reload so that is a consideration. During the ammo crises the last few years, I would see 350 ammo on the shelf everywhere, it is not popular here in Oklahoma, I thought about buying one just for that reason, lol.
The 300 BLK was a cool concept. I started with it when it was still the 300 Whisper and there was no commercial ammo for it. I loaded everything from 30 cal carbine 110 grain round nose, the 125 grain spitzers to 208 grain Amax for it. I shoot all of the ARs suppressed most of the time. The 300 is a mild round really reminds me of the 30 cal carbine. I lost my interest when I bought the 7.62 x 39 and saw the power difference with the 150-154 grain soft point ammo. Not to belabor the point but many people are confused about the 300 BLK vs the 7.62 x 39, so here is my point for hunting uses. Most people like to have about 1,000 foot pounds to kill a big deer or hog or black bear. Here are two of the Hodgen charts showing the bullet I use, a 150 grain Hornady BRSP with the BC or .423. There are hotter recipes but Hodgen shows enough difference to make the point. You can load a 300 BLK up to about 2,000 fps. Note the chart. The BLK actually holds the 1,000 fpe to 150 yards. The 7.62 x 39 holds it to over 250 yards, a full 100 yards more. So basically, the Russian round is good for hunting a lot farther. That is why I like that round in the AR platform. It is a true 200-225 yard deer gun if I want it to be. The 300 just does not carry the power very far.
If I were building to day, and already have one in 5.56 and one in 7.62 x 39, I would definitely go with Wicked109, the 350 Legend has lots going for it. There is a ton or reloading recipes out there and you can use any 357 pistol bullets for fun or varmints. And it is superior to the 7.62 x 39 for hunting at ranges out to 200 yards. During the ammo shortage the last 3 years, I could find 350 ammo on the shelf all the time, even at Walmart who does not sell assault rifle ammo, lol.
I have no experience with the Grendel. I do have an interest in the 450 Bushmaster. A buddy has one and it is a cool round. Brass is hard to find at times, but Starline has it today at 74 cents per round. The 450 has limited use for me, I have a 45-70 to fill that role. But it is definitely interesting.
Let us know what you decide, and then give us some feedback down the road.