If you are going the mail order route and you want a shooter ... order the Service Grade (SG). If you have no preference for manufacture, get the HRA Service Grade. Most of the SG HRAs I have seen lately were good looking and shooting guns.
Of course your best bet is to visit the CMP South Store at Anniston, AL and pick out your Garand. If you are lucky, you may find a field grade with a good barrel. A lot of the field grades I have seen have terrible crowns and accuracy can suffer big time. Just remember, a field grade gun would NOT be issued to the troops while a service grade gun would be.
As SGF stated above, the lower the muzzel and throat gages, the better. I have purchased my fair share of garnands from CMP in all grades ... I highly recommend the HRA Service Grade. If will cost you a $100 more but well worth it IIHO!!