The only definite reason to get a trailer is if your bike can't mount racks and you have too much stuff to carry in frame/saddle bags.
I've used a BOB and I've used panniers. There is nothing wrong with the BOB, but I prefer panniers.
Two wheels vs one wheels? If you are hauling a week's worth of groceries a two wheeled trailer will have more capacity and be marginally easier to load up. If you are touring, a BOB will track your bike so there's no worry of a second wheel getting caught up on something.
Trailer vs panniers is one of the classic threads done multiple times. What frustrates people is that it would seem that there should be enormous differences between the two methods. But both work.