For me it depend how much weight I carry and the weather. In south Florida, more than 20 min with a backpack in the summer time and you would know why I went to panniers and trunk system. Going to say this one more time, the ortlieb panniers and trunk are worth every penny. Believe me, save your money and buy one of you ever decide to go this route. I went trough 3 different trunks and 3 panniers before I bought ortlieb's. The Cannondales where the better ones and used them for a long time, and if I didn't know better I would have recommended them.
When I got the ortlieb trunk, I was so impressed, that shortly after I got the panniers. This system is so versatile, that I use it on my two bikes. I got the front panniers that I carry on the rear rack, on both the gravel grinder/commuter and the road racing/alternate commuter with no heel strikes.
Change of socks and underware, take trunk only. Fast and fun ride. Need change of shirt, breakfast and rain gear, take one panniers and the trunk. Need full change of cloth, some lunch, I pad, and others, take all three. Need to get on and off the train, the system allows for ultra fast on and off the rack in seconds, if not in milliseconds. Need to carry the stuff walking in the mall, I securely lock the trunk to the rack and take the pannier with me on the back pack system you can buy for them. No more sweaty hot back or computer poking your skin. It has made child play of transporting stuff to from work.
Recently went on a 4 day tour and that's all I took, 2 front panniers, carried on the rear rack, and the trunk. No tent or sleeping bag, but I had space for more if I needed to.
Back pack I used for almost a year in south Florida, and the months that are cool here ( Jan, Feb) if the load is light and soft it's not bad or that uncomfortable. You start adding computers and lunch and others and before you know it it's close to 20 lb on your back, making you more cautious on the turns, ending with a hot back and a very unpleasant long ride.
That's my 2 cents!
Double O