As has already been said, it depends. In my experience, it depends mostly on how much Fine British Weather you're expecting, and whether you're carrying camping kit (and how posh said camping kit is - cheaper tents, sleeping bags etc tend to add a lot of bulk, if not weight) and/or cooking kit.
For a couple of nights' camping with a stove, my preference is for four lightly packed panniers rather than the two plus tent strapped across the top of the rack that I strictly need (I have a cheap rectangular sleeping bag because mummy bags give me back pain, and it fills most of a pannier by itself). It distributes the weight much more evenly, which is beneficial when you're hefting the bike about, as well as when climbing hills. It's also handy to have some spare room in the panniers for taking on shopping or extra water or whatever (though a very basic shoe bag/rucksack is a good alternative for that sort of thing).
I already have bikes with front racks and nowhere sensible to mount a bar bag, though. Front racks are very often a pain to fit (I had to manufacture custom brackets to fit the fork on my Dawes hybrid), so it's quite reasonable to want to avoid them. Two panniers and a bar bag seems to be a popular combination.
If you're not camping, then it's really just spare clothes (so a function of weather and whether you need to not look too much like a cyclist in the evening), bike tools, bonk rations, camera, wallet and so on. You can fit that in pretty much anything. My preference would be a pair of 'front' sized panniers, but bar bag and saddle bag / rack pack / pannier would also work well.
A useful skill to develop is the ability to resist filling your luggage with stuff you don't need. If you lack that skill, you're better off with smaller luggage. I'm terrible for carrying too many tools and quite a lot of food (though that's largely justified, I can't rely on finding stuff to eat at random pubs or wherever), but I have now learned to leave the kitchen sink at home. Rumours that I have climbing ability are greatly exaggerated.
I highly recommend the use of
semi-waterproof stuff sacks for managing the contents of your panniers, especially if they're Ortlieb-style single compartment ones, or less than optimally waterproof. Saves an awful lot of rummaging around, and lets you keep the wet clothes separate from the dry ones, and so on.
