Before choosing bags, lay out the kit you intend to carry and consider the route. A longer trip with camping gear and food usually calls for more capacity; a short ride on rough trails may favour a narrower bicycle. That difference is a useful starting point when choosing between rack-mounted panniers and bikepacking bags.
Panniers can divide a larger load and make belongings easy to reach at a stop. They do, however, need a compatible rack rated for the weight. They also extend beyond the sides of the bicycle, which can be awkward on narrow trails or when you have to push it.
Frame, handlebar and seat bags keep the bicycle slimmer and often work without a rack. Their capacity is more limited, and packed items can take longer to reach. Before setting off, check the attachments, clearance from tyres and legs, and the effect of the load on handling. A heavy load should not simply be shifted to an oversized seat or handlebar bag.