The biggest difference is that a real M21 is based on an M14 receiver, which you're not going to find for less than $10k. The original ones used a Leatherwood 3-9 ART scope. It was advanced in it's time, but difficult to use. The mount was a single point mount on the side of the receiver where mounts available now are two- or three-point.
What is being offered by Springfield Inc. is as close as you're going to get to a real M21, but I'm sure it comes with their scope mount, which has a poor reputation. They don't mention the scope that's on it.
Real M21s may have started with the standard wood stock, but the Army did switch to McMillian stocks later on. Springfield's is rear lugged and bedded, but the bedding would need to be refreshed to keep accuracy at its best.
Their M25 looks like it's a step up and uses a rail mount instead of the standard scope mount.
As Jason mentioned, you're comparing a military rifle to a civilian copy. They can't directly be compared, but you are not going to find a real M21 on the civilian market. If you built one up from a receiver, you could make it closer to a real one than Springfield does, but it would be even more expensive. And, the M21 was changed over the years with the McMillian stock, a better scope mount, and better optics.
Unless you're looking for something that resembles an M21, I'd look for a standard M1A, add a J. Allen stock, ARMS mount, and a good scope. This would be a good foundation for a distance shooter.
http://www.jallenenterprises.com/
