I have both and like both. I like the trailer for longer items and tougher terrain. It is nice not to have the weight always on the bike. I have to keep the heavier items as low as possible or it will develop a shimmy. Two things to be aware of, it is difficult to change the rear tire because of the hardware for the trailer. If I am traveling and need to use a train or airplane, shipping costs more. With the length of the bike and trailer, it is long and one must remember it is back there.
I have a complete set of Ortlieb Roller Classics Panniers. They are a delight to use. Easy to get on and off, durable, adjusts to different size items, 5 year warranty. What I like the most, they are completely waterproof, no rain covers. The two things I find slightly negative, since they are top loaded, some people like side compartments for easier access and organization. When my tent, drop cloth and sleeping pad are on my rear rack, there is no access to the rear panniers. When riding with loaded rear panniers, it helps to use front pannier to balance the load. Like the trailer, keep the heaviest items as low as possible in the panniers otherwise a shimmy may follow. I use an inexpensive hanging fish scale, from Walmart, to weigh my bags to make sure the weight is even. Good luck.
