A normal rack is better. The load is lower and less likely to sway. Also if you have a level top tube, the seat post rack rides too high under the seat making less room for your stuff on top. If you have a sloping top tube it's a little better since the seat post is longer and the rack rides a little lower. People say to avoid seat post racks if you have a carbon seat post.

You'll need to check if your bike has rack eyelets on the dropouts. Otherwise you'll need "P-clips" to attach to the seat stays.