I'll contribute here since I have also been curious about the relative merits and recently used both four panniers and then a B.O.B. for consecutive legs of the same tour and was surprised at how much of a tossup it really is between them. I had previously toured extensively with rear panniers and handlebar bag, and expected adding front lowriding bags to be an improvement. I did like my plan to use the larger rear bags simply as stuff sacks; right for the sleeping bag and left for tent, footprint and fly, and the collapsible REI camp chair that was delightful to have in camp. My clothes and food went in the front bags. I tried to trim out the excess and still wound up with 50 pounds hanging on the bike. I used this setup for about 9 days on the road and my buddy's bike was set up the same way.
My buddy had trouble with his front bags so we relaunched with him using a pair of my panniers because I took another friend up on his offer to use his trailer. I put the same contents into the B.O.B. bag that had been in my panniers. Same content weight (plus one spare 16" tube), minus pannier weight, minus front-rack-and-lowrider weight, plus trailer weight, so maybe I netted a little more weight on the second leg but I was carrying the same stuff differently.The handling of the bike with the four panniers (and handlebar bag and rack top pack) seemed ponderous. It felt like it was a lot of weight but the handling was not bad, just...loaded, the way touring bikes feel. I did get the unsettling shimmy on descents that I have experienced on other bikes and which I really do not like. Climbs were tough and I walked a lot of them despite adequately low gearing. Access to the packed items was good with the panniers once I memorized where everything was.
The trailer generally felt more stable and I never got the feeling on descents that anything might go wrong but I tried to keep my speeds down. Trailer stability felt like it responded to weight distribution. Having heavy items on the bottom rode better, but my heavy items were the food bag that I wanted easy access to, and the tent that would be the first thing out when stopping for the night, so keeping these on top was handy for me at the risk of marginally less stable riding, an annoying tradeoff. Climbing seemed equally difficult with either setup and walking either one was a lot of work. Standing while climbing was slightly easier with the trailer.
The one large bag on the trailer was more trouble because it was not compartmentalized as separate panniers are. I already had all contents in plastic bags and this was useful when opening the large bag in the rain, all that stuff was protected. Trailer probably had capacity for even more stuff if I needed to carry it, but more weight would have simply killed me. I was surprised that the contents I carried filled the trailer bag; there was not much extra space in it for anything else, just as I could fit nothing more in my panniers without cramming. I had enough stuff.
I also found the ESGE Twin kickstand very useful on the bike in both its configurations. I'd used conventional stands previously and that worked for rear bags and HB bag but the bike needed the Twin with the front panniers added, and it was a help during the trailer phase also. I had also used a rear stand from a commuting bike and that was as useless as the center-mount on the fully loaded Bridgestone RB-T.
The bike/trailer setup had two stages with some discoveries included. At first I rode with no bags on the bike and everything stuffed in the B.O.B. bag as my friend does on his tours, but I discovered that with no additional weight on the front when you stop on a hill and use the front brake as your parking brake the wheel will be dragged downhill by the weight you're trying to hold immobile. Having the handlebar bag on the front mitigates this but subsequent riding with the handlebar bag and the rack top bag diminished the liveliness of the bare bike and made towing the trailer more of a chore. This setup does have benefits; the additional weight on the bike makes it less likely to tip if you happen to lean the bike and trailer combo too far to one side. The leaning trailer wants to flip the whole rig or jackknife at a standstill but the weight of the two bags high up on the bike make it less resistant to the tipping force. Just one of those weird things you would not anticipate.
If I were taking off tomorrow it would be a tough decision. I prefer the idea of touring with panniers because that is what I'm used to and it fits my philosophy but the trailer definitely works, maybe just not better. And I would have to leave a lot a home to make the load noticeably lighter in either case, but that might be the best plan.