I have the RCBS Gold Medal and the RCBS Comp Seater
Both let you insert the bullet in the side into an opening and it falls down in it's collet which holds it in line with the neck of the case (WARNING: very addictive, once you get used to it you will hate using the others). The bullet starts straight and is easier to seat, less stress on the neck brass. I also have some Redding Competition Seaters which I get when I can't find one of the RCBS Comp Seaters and it takes more force to start the bullet seating than with the RCBS.
The Gold Medal RCBS differs in 3 ways from the RCBS Comp seater
1. The bullet collet covers the neck, bears on the shoulder and comes down 1/4" down the case body (the Comp seater only covers the neck and shoulder)
2. The seating stem is stainless and floats up and down unlike the Comp seater which has a fixed black seating stem
3. The Gold Medal somehow will hold the bullet and not let it fall through. IOW if you drop a bullet in the side window of the Comp seater it will fall through unless you have raised the ram high enough to hold it in place (convenient but not necessarily necessary)
Personally I have chased concentricity extensively and I can tell no difference in the final product between the Redding Competition Seater and the RCBS Competition Seater and I love the side window.