I like the backpack better as I can bring it with me on the last 100 ft and into my building where I shower and change, where if its pannier's I either need a backpack inside of it, or I need to carry my stuff around.
Also people steal stuff, so I don't like leaving things on my bike.
There are pannier backs with backpack straps, or shoulder straps, etc.
Although I have the more basic "back roller" standard detachable panniers. They take a second to attach/detach, nothing remains to be stolen. Best £50 I ever spent! They still look new after 2 years of daily use.
Also people steal stuff, so I don't like leaving things on my bike.