Simple answer to OP's specific points in his question... Yes, 6.5 Grendel have a flattery trajectory and is more accurate. However, .308 is flat enough and accurate enough for any purpose you are likely to need to use it for out of an AR platform. It is also a larger, heavier, and more powerful cartridge with all the benefits that provides for hunting, combat, and defensive use.
So basically if your goal is target shooting, get the Grendel. If you goal is almost any other use, general use, or mixed use, then .308 is just way more versatile..... And of course a fair bit cheaper to shoot too, and if you intend to shoot much at all ammo savings will quickly exceed hundreds of dollars of extra up-front costs.