The .243 win is shot in a AR10 platform and cost waaaaayyyyyy more than a Grendel which is shot from an AR15 platform. The AR10 platform is heavier, more expensive and there are less factory parts available. Other than that, if you have the money it's a good platform. But, if you're gonna pay for it why not just go with a .308??
As for a Grendel. I have one and I love it. It shoots very flat and stays supersonic to 1200 yards. It fits into the AR15 platform and only takes a barrel, bolt and magazine change to convert a 5.56/.223 into a Grendel. It shoots better than the 6.8 out past 350 yards. I'm actaully considering building a 6.5 Grendel on a Remington 700 SA to use for varmint and coyote.
People are taking deer and elk with the Grendel, so I'm positive it will do just fine for coyote. I just haven't had a chance to try mine out yet.
You can find tons of good info about the Grendel on the site ..........