Almost done with this AR15 pistol build and am getting prepped for the next rifle build. Got the M4orgery for SHTF and will have a .300 BLK suppressed AR15 pistol with Sig brace to replace the shotguns that have already hit the consignment shelf at the LGS. So, this will strictly be for hunting hogs from an elevated blind or tree stand. I'd like to use the heavier 154 grain SP for the deeper penetration. It is my preference to overpenetrate 50# shoats and reach the vitals of any >500# boar which is getting more common around the family farm; than, to use 123SP to still overpenetrate 50# shoats and just piss off a rather largish boar.

Generally, all the 154 grain bullets are listed with a 2100ish fps MV with a 16" barrel. Guestimating a 50 fps/inch increase, I am expecting a MV of 2300 fps with the 20" barrel. From the Hornady Ballistics Calculator:

150 yard zero
2100fps MV, +1,9 at 100 yards, -4.6 at 200 yards, with 1774fps, 1076 KE remaining
2300fps MV, +1,5 at 100 yards, -3.7 at 200 yards, with 1955fps, 1165 KE remaining

which highlights the rainbow trajectory of the 7.62x39mm round. Another interesting point was a thread(have to refind and post), was that someone had reloaded the 170 grain bullets for the .30-30 in a 20" barrel 7.62x30 AR15 and was getting 2100 fps out of it. Now that is intriquing!

My question is for a hunting AR15, would it be better to get a 20" barrel or stick with the 16" barrel?

I prefer the shorter barrel while hunting. Easier to handle and still deadly. I doubt the hog will notice if it's 2100 or 2300fps that kills it.

I have a 16" 762x39 AR. The barrel is Surplus Ammo and Arms, which was not high dollar. I have about 200 rds. of Herters 122 HP through it and the Herter's is amazingly consistent in velocity. The 12 " gong at 400 meters from a rest is slightly problematic if the breeze is fluctuating at the range.  Of course, That is too far to shoot at a hog as energy drops like a rock after 200 yds. with the 122 grainers. Just pointing out the accuracy of cheap ammo from Cabelas. I have some Herters 154 gr. SP,  but have not sighted in with it yet.
At your given 200 yds, the 154 SP will do the job on a big boar if you hit the vitals, even a bit farther with a 900 ft lb cutoff. I would go with 16" if on a stand or confines of a blind.

As far as handloading there would be no gain in a 170 gr. 30-30 bullet if it is flat nosed. There probably isn't enough real world gain even if you loaded a premium 150 gr. hunting bullet either to bother with handloading since the 762x39 has limited velocity.
Then you have to consider actual bore diameter. 762x39 barrels can vary. That opens up a whole nother can of worms.  Saami says -.002" is ok. i.e. a 308 bullet can be used in a .310 bore. but that begs the question of accuracy. Lee and Speer manuals spec a .311 bullet. I'm not sure who makes a premium boat tail .311 hunting bullet that may give a bit more range  ((about 25 yds farther with a 16' and 50 yds with a 20" using 900 ft lbs of energy as a cutoff) vs. the Herter's 154 SP.
Just my 2 cents.
Happy Hunting!