drmweaver2

Rode one 1300 mile tour using both trailer and panniers (not at the same time). Short and dirty summary:

Trailer (BobYak semi-clone/single-wheel)
- less affected by crosswinds
- technically able to carry more/heavier/strangel-sized/shaped loads
- carried 1 additional inner tube (trailer wheels not the same size as the bike's)
- handling issue #1 - bike & trailer got tipped over by a rambunctious kid. Picking it back up was more awkward than doing the same thing with panniers
- handling issue #2 - this particular trailer mounting system was more sensitive to the QR skewer/mount being "just so" than any pannier mount I've seen/used. Incorrectly tightened just slightly, it caused a crash with minor injuries when torgueing the pedals while starting off (occured pre-tour during a warmup/shakedown ride).
- handling issue #3 - seemed to take longer to stop than with panniers
- reduced weight load on rear axle
- far larger turn radius/harder to back up
- no heel-strike issues due to panniers mounted too far forward
- extra locations for mounting water bottle cages
- extra wheel to check pre-ride each day (air pressure, detritus in tire, axle tightness, etc)
- 1 "big" bag rather than 4 smaller ones (storage space and carry issue - could go either way depending on how loaded/stuffed)

Panniers and racks (front and rear)
- sensitive to crosswind gusts affecting "my line". Definitely noticeable when winds were 20mph+ - but still usually more than manageable.
- more sensitive than the trailer was to load balance both left/right and front/rear
- much more built-in organization of packed items
- weighed slightly less than the trailer (negligible in terms of being rideable)
- three more bags to waterproof than with trailer cargo bag
- easier to park than the trailer
- daily bolt check tightness routine more involved than with the trailer. Did have to retighten one pannier mounting clip during the 3 week tour (have heard of others who've broken/had to repair mounting clips). Have heard of people losing a pannier at very inconvenient time - have never heard of similar issue with trailer cargo bag.
- wider than trailer (straight-line path wideness issue)
- I found panniers easier to balance at <5mph than the trailer
- needed to match rack to pannier mounting "system" (usually not a big issue, but I used "no longer made" panniers and a decades-newer rack).
- removing rear wheel/repairing rear flat was more time-consuming than when using the trailer

Others probably have difference experiences/perspectives.

Preference - shrug - it's a coin toss depending on terrain, tour length and road conditions. Trailer in rolling hills seemed to require more effort than panniers. Garbage-strew/rough shoulders were easier to negotiate with the panniers. In windy conditions, I definitely prefer the trailer.