Neither. a shoulder bag with a very long strap so that when you are on the bicycle, the bag is below your waist. This keeps the center of gravity very low, so the load doesn't flop around like it does with a backpack, and also keeps your back cooler since the bag is hanging off your waist and not stuck to your back.
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Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1