Accuracy difference between the two could not possibly be more the 2" at 25yds. And then that would be very ammo dependent. What shoots well in the Glock may not shoot as well in the HK, or vice versa. So I would work a load up with the Glock that made it more accurate. But even a 2" change at 25yds (which seems unlikely) is not going to change much in a gun game shot mostly at 15yds or less. I would never use a IWB holster in competition unless I just "wanted" to.
For the record. My G34 can shoot 10 rds into 4 inch's at 25 yds. Freestyle. No bench. Home cast lead, not sorted brass, not specially prepared ammo. End of the day of shooting so I was tired and not shooting as well. If you throw the flyer out it was 2.5 inchs for 9 rds. You really can't expect much more then that from a service pistol. I do have a KKM barrel but it's not some fancy fitted barrel.