Both manufacturers made solid rifles. Springfield produced M1s from the 1940s through the 1960s, so you may get a WWII rifle. HRA didn't start manufacturing until the 1950s, but their rifles have always been considered the nicest for fit and finish. Service Grades will be mixmasters but will be solid rifles and there is a chance that you will receive a cartouched WWII walnut stock. I have a few service grades and consider them one of the best values that a shooter/collector can find. You cannot find them at a shop or gun show in Service Grade condition for less than $900. The Service Grade Specials will have (relatively) nicer metal, but they will have new production stocks and stock metal. They will be very nice, but I personally much prefer the USGI stocks as they have character and a story behind them. I would actually choose the Service Grade over the Special even if they were the same price (Special is presently $300 more). Also, because it is luck of the draw, I have friends who have ordered Service Grades and still received the new CMP stocks.
You can write any special requests (USGI stock, WWII receiver, etc...) on a sticky with your order and they do their best to accommodate. Good luck.