OP is too vague of a question to be answered properly(i am ignoring the shit shot of the last few pages). Both are fantastic cartridges that can be pushed well outside off their intended role, but in my opinion the 6.5 Grendel does more things "I" want. Your mileage may vary.
The 300 blackout is a fantastic Subsonic round. Since it uses rifle bullets with high B.Cs it has a far superior range to any subsonic pistol cartridges. Can you push it to great distances? Sure you can! Is it the ideal cartridge to do so, hell no! But sometimes that is the fun of it! Look at all the Creedmore matches with 45-70, that is fun!
The 6.5 Grendel is more of a long range cartridge that was designed around the length limitation of the AR15 magazines. It has fantastic brass life, 25+ reloads with Lapua brass out of the AR15. Even more so if you have a bolt gun and only neck size. It makes a fantastic hunting cartridge with very low recoil. There are subsonic loads for the Grendel, but you have to reload for it. They use the 160gr bullets. Which don't have the BC of the heavy 220+gr bullets that the 300blackout can use.
When i built my first AR, i was just starting college so i didnt hvae a bunch of cash to be able to build multiple AR's. So i built one in 6.5 Grendel as i felt it gave a good mixture of all the things i wanted. Long range shooting, hunting, Home defense(not the most ideal since it had an 18 inch barrel, but it wasn't bad for me), etc.
I will comment on this. The whole bolt breaking issues for the 6.5 Grendel was dumb. I only came about became AA was in the middle of trying to get the round SAMMI speced and people didnt want to wait. So they came up with similar chambers but didn't use the proper bolt. So that did give the Grendel a bad rep in the beginning.