what difference in Ballistics? With a 200 yard zero, At 300 yards the Grendel drops about 1.5” more than a 243 and has the same energy.
When it comes to killing deer the Grendel will do everything the 243 will, and will do it with less recoil. Which is why it’s a better low recoil deer cartridge.
Alternatively, if you run the numbers for MPBR on both, you will see the 243 has a clear advantage. I feel that MPBR is the best way to sight in a scope for a new user, because then as long as the shot is within your MPBR you just point and shoot, no need to consider things like drop. The fact that the MPBR on the 243 with a 6" target is damn near 300yds is almost perfect IMO, because if you need a rifle with such low recoil you probably don't have the experience/skillset to shoot beyond 300yds anyways.
95gr 243cal Ballistic Tip, 3050 FPS MV (which is 110fps lower than the max load in Nosler's data), target size=4": MPBR= 253 yds. Target size=6": 293 yds.
120cal 264cal BT, 2520fps MV (max load from Nosler), target size=4": MPBR=215 yds. Target size=6": MPBR=250yds.
So we're talking about 40yds farther MPBR for the 243, a bit more if you're loading it to the max of Noslers data (3160fps, MPBR=303yds on a 6" target, or a solid 50+yd advantage)
At the end of the day both will kill deer just fine and we're splitting hairs though.
edit: For ####s and giggles, I checked Chuck Hawks' recoil table for both, and as you can see below they are very close with the same weight rifle:
load - gun weight - recoil energy - recoil velocity
.243 Win. (75 at 3400) 8.5 7.2 7.4
.243 Win. (95 at 3100) 7.25 11.0 9.9
.243 Win. (100 at 2960) 7.5 8.8 8.7
6.5mm Grendel (120 at 2600) 7.5 8.9 8.8
6.5mm Grendel (123 at 2590) 8.0 7.9 8.0
Now, that 243 load is a tad slow for reloads (although bang on for factory ammo) and that Grendel load a tad fast compared to the numbers I ran above, but clearly you can see that the difference is not big - in fact, with the 7.5lb rifle the Grendel has ever so slightly MORE recoil, although not enough of a difference that anyone would notice. Looking at the numbers, I agree that by and large they'll do the same thing. I'm not so sure about the recoil difference being enough to be a factor though.