Panniers vs trailers is a frequently discussed topic both here and at CGOAB. It is really a matter of personal taste.
For the 35 to 45 lb load I generally carry, I prefer the way my bike (Surly LHT) handles with 4 panniers over the way it handles with a BOB.
To me, the BOB feels like it's dragging the bike more than panniers on the uphill and pushing it more on the downhill. The BOB is always a pain to maneuver in tight quarters. Someone else might dislike the way front panniers affect the steering response, but not be bothered by the trailer drag and push.
OTOH, to me, the BOB seems to have less resistance in a head wind than panniers. And if I need to carry a lot more weight, as on a run to the store, or have stuff that exceeds the volume of what comfortably fits in the panniers + on the rack, I'll use the BOB.
In terms of locking things up, I don't think there is a significant difference between panniers and BOB. A determined thief, human or otherwise, can get at whatever is inside fairly easily. Keep your valuables in a bag or pack you can take with you when you leave the bike.
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