Another thought is about access and separation. If you have a rubber boot and you've packed your socks on the bottom, you have to rummage through or take out everything top get your socks. Panniers with a 180 degree double zipper allow you to open a hand size space at the area you need and retrieve what ever it is you need without disturbing other items and requiring a re-pack. Also lots of pockets means that you can separate items more easily.

Everyone has their favourite pannier but as a rule the ones on the front are smaller than the ones in the rear. Pack heavy items low for maximum stability of the ride.

Enjoy!