Originally Posted by
cooker
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.
I learned all this the hard way, he's right. The lower height of the rear rack makes it easier to get at the in the bag and makes the bicycle more stable. And it's easier to get on and off when the bag is lower.