Let me make it simple for you......GET BOTH!!!
Kidding aside it's hard to beat the GRS Crown if you are going to spend most of your time shooting bench or prone, the adjustable stock is phenomenal except it doesn't have the butt pad height adjustment. Grant that's really not needed for airgun with zero recoil but certainly nice to have being a trainer and at that price point.
Qball is right! Get both.
Seriously, I happen to have both. I also happen to love them both. They are great guns. They are both equally capable shooting pellets out to 100 yards. My Crown MK2 in .30 cal is actually more powerful than my Mk2 Impact in .30 cal. Now, I currently have my M3 set up as a .22, and there is no question that the M3 (and the MK 2) can both be adjusted up in power a bit more than the Crown can, but for pellets this doesn't really matter, since either gun can drive just about any pellet faster than is advisable. For most slugs that is also true, but at the end of the day if slugs are your prime consideration then the Impact does have more "upside". This is particularly relevant since some slugs seem to like, or need, a higher speed for optimum stability. Given that slugs also tend to have greater coefficients of friction in a barrel (greater than pellets that is), then you can make use of the extra power and adjustability in the Impact to find that sweet spot. For probably 98-99% of shooters this will not really matter, but it will for some.
For me, slugs just add the complexity of overtravel and that is something I don't need. Generally 99.99% of my shots are inside 100 yards, probably 95% inside 75 yards, since that's as far as I have in my property. I shoot the Crown offhand really well, but the Impact is very close. The Crown is easier on a bench just due to the form factor, but the Impact cycles and shoots faster (i.e. I can pull the trigger faster) than the Crown. One other point, for maximum power you need a 700mm barrel. A 700mm barrel on the CRown is very long, especially when you add a moderator. It's not a "walking around" gun. The Impact is much more compact, even the 700mm gun is not really long.
The Crown does have a superior trigger......
Did I mention I love them both?
Which ever one you pick, you're gonna love it.
Chris