The reason I suggest a rack is because it is a lot more versatile than a seatbag. With my rack, I can choose from any number of types of bags such as plastic grocery bags, small duffel bags, dry bags, trunk bags, Carradice bags, panniers, or whatever I happen to feel the desire to use. I can even bungie down my jacket and other items without any bags at all.
But I do use a Carradice saddle bag for many of my rides ... and I find it is more stable and sits better with the rack underneath holding it up.
