I don't know if the picture will show it
but on the one on the left the sliding collet comes down over the neck, shoulder and a portion of the case body (it is about 1 1/2" long). It holds the bullet in the collet also. So when you load the bullet in the side the bullet and the case are aligned before it starts in the neck, unlike any other seater in which you have to balance the bullet on the neck and then raise into the die. That is a better system for starting the bullet in the neck.
I have Redding Competition Seaters and RCBS Gold Medal Seaters and I can tell you from experience that they both do a good job. From experience, since I have both kinds, I can say that the RCBS Gold Medal Match is just as good if not better than the Redding Competition Seater.