I think the 6.5 grendel would be inherently more accurate.... I have not owned a 243 for many years but my current Alexander Arms upper in 6.5 Grendel on a AR platform is an absolute tack driver. I get about 3,200 fps with the 85gr Sierra HP's and they shoot .5MOA all day long. It's the perfect coyote hunting
I would think that for a truck gun a bolt action may be more appropriate and this might steer you to a 243 or a 260 remington.