The question of backpacks versus panniers was covered by "Bicycling Science" MIT Press - 3 Editions now. Weight on the bike is less stress, then weight on the back and the rider with backpack has worse balance and a higher center of gravity. The lower the weight on the bike the lower the center of gravity, and the easier handling the bike. Rear packs without front packs causes servere handling problems due to the light front end, poor steering and poor aerodynamics. All panniers should be loaded tight or strapped tight to give the best balance. Front packs mounted low at mid axle improve the aerodynamics of a bike and the balance. Front packs cause slightly slower steering. Seatbags have little effect on aerodynamics. Small aerodynamic low front panniers with a large wedge shaped seatbag give decent balance and great aerodynamics. Carry a wide strap to hold it all together when off the bike and throw the load over your shoulder. The newer mounting systems are much more secure and each pack can be removed in about 2 seconds. I put my heavy bike lighting system battery in the seatbag to balance the front low rider panniers. Unless I am doing a major load I don't use rear panniers. My Trek 410, Sport Tourer, has only the front racks. I switch my seatbag with battery and NightRider light, each bike has a slip in headlight bracket, harness wires and I have a jack on each seat post, when I go to my MTB. My MTB has a rarely used rear rack, but I have an extra set of panniers, but for real loads I use a Bob Yak bike trailer - the bike with the trailer handles poorly without front panniers - so I always load the fronts when using the trailer.