something to consider is the weigh balance of a bag that is high and behind the saddle, especially if you are considering putting much weight into it. This is not quite as much of an issue for a smaller frame size, where everything is closer to the "axis", if you will. I have had horrible luck with any saddle/seatpost mounted bags, particularly for any "out of saddle" riding such as on hills or to pick up the pace on the flats.

For a lighter/faster setup (typically Summertime for me) I actually prefer wearing a backpack to the tail-wagging sensation that I get with a saddle/post bag. Again, this is largely due to the fact that I'm 6'2+ and on such setups the weight is way out from the axis of rotation, making for a very unpleasant dynamic.

If you are on a smaller frame (say, 56cm or smaller) then this may not be nearly as much of an issue.