The difference is the offset of the arm. Look like you chose a V rack which will raise the height of the bag. You may or may not find this a problem with having to raise your leg over the bag to get on your saddle.

I do own a V rack for my FS MTB because everytime the rear suspension dip down from a bump, the tires rub the bottom of the rack. The V rack gave me more clearance to prevent that. When I used the sane rack on my front suspension bike, it placed the bag higher than the saddle and also the effort to mount on the bike was mire difficult.