Get the service grade. A buddy that shoots with me at Butner every year got an "expert grade" for shooting Garand matches and it shot like complete crap. And by crap, I mean an average of 8"-10" groups at 100 yards using Creedmoor Sports Lapua 167gn match ammo. CMP refused to accept it for return (they are sold as "no returns"), but still blamed it on being assembled by a "junior armorer" who did not know what he was doing. He spent quite a few dollars getting the stock and both handguards fitted correctly by an old WW2 Marine Corps armorer (which fitting is supposed to be done by CMP) and it was a little better, but still shot 6" groups at best at 100. He eventually ended up having to swap out several parts (gas cylinder, gas cylinder lock, entire rear sight assembly and some other internal part) to get it down to a 3-4 inch group at 100 yards with match ammo. Personally, I think somebody was being lazy and sent him a "rack special" instead of an "expert".
That said, buying a service grade does not guarantee you will get a rifle that shoots good groups. Nor does buying an "expert" guarantee that you will get a lemon. But at least with the service grade, you know you are not massively overpaying for a rack grade rifle with an unfitted stock and worn out parts. I've personally always gotten good luck and service from CMP, but I would not take the risk on an "expert". YMMV.