Panniers excel on paved-road riding and single-wheel trailers are at their best on rough, unpaved terrain, but both can work well for nearly all types of touring. Personal preference is the ultimate arbiter.
Here are a few articles to get you thinking:
If you trip includes travel by air or bus or train, a trailer may involve extra effort and/or expense.
Obviously, if your bike won't take panniers well (full carbon, short chainstays, no fittings, etc.), you will probably use a trailer.
The ultralight people will say the answer is "neither".
For me, I go with the wisdom of crowds.
This is more religion than science.