You can't compare a .45 pistol round to a .30 caliber rifle round. The ballistics are far different. The .30 cal rifle round gives you a huge ballistic advantage, far more loading options, and can be used at longer ranges in all weights.
Actually it is pretty easy to compare them. You punch the numbers in to readily available software and it give a pretty good bases for comparison. If one is talking about supersonic loads yes the 300 BLK is notably more powerful, has more practical range, and better terminal ballistics.
The post to which you responded was comparing sub sonic loads. That is a very valid comparison.
With a 220 SMK bullet with a very favorable .629 BC fired at 1050 FPS it will drop 16.73” at 100 yards and 66.93 by 200 and 152.24 by 300. At 200 yards it has 483 ft/lbs of energy. It should also be noted that the SMK most likely will simply punch a 30 caliber hole and offer pretty horrible terminal ballistics.
A 230 grain Gold dot 45 caliber bullet with a .143 BC, fired at the same muzzle velocity, drops 17.94” at 100 yards 76.16” at 200 and 182.6 at 300. It has 366 Ft/lbs of energy at 200 yards. Importantly, for anything other than punching paper, that gold dot is going to offer vastly better terminal ballistics.
This is a little simplified and there is more we could look at; I encourage you to crunch the numbers yourself. However, when comparing sub sonic loads what I see is that out to 100 yards the trajectories are close enough that it doesn't really matter. However, the .45 would have much better terminal ballistics and components to load the 45 are cheaper.
By 200 yards the advantage of the high BC bullet is showing its self in trajectory. However, there has still been a ton of drop and a lot of drift. If one can correct for 67” of drop (33" lower impact w/ 100 yard zero), is correcting for 76” of drop ( aprox 40" lower impact with 100 yard zero) instead really a huge deal. There is more drift and thus errors in reading wind are likely to be more pronounced. The 300 BLK would be carrying more energy but the terminal ballistics, are with all likelihood, much worse than the .45’s.
This is why I fail to see the big advantage of the 300 BLK for subsonic shooting against anything more than paper (although maybe there is something I am failing to account for, if so hopefully someone will educate me). The comparison is easy enough to do, and out to distances that one is likely to shoot subsonic rounds (say 150 yards), there is not a huge trajectory advantage and the .45 has better terminal ballistics. Inside of 100 yards I see the 300 blk as having no advantage and yet costing more, and having very poor terminal ballistics.
Again if 220 grain 30 cal bullets that offered both a high BC and good terminal ballistics at sub sonic velocities were available the discussion might be slightly different.